SW Sextantis stars: the dominant population of CVs with orbital periods
  between 3-4 hours
Mon. Not. R. Astron. Soc. 000, (2007) Printed 27 October 2018 (MN LATEX style file v2.2)
SWSextantis stars: the dominant population of CVs with
orbital periods between 3–4 hours
P. Rodŕıguez-Gil1,2⋆, B. T. Gänsicke2, H.-J. Hagen3, S. Araujo-Betancor1,
A. Aungwerojwit2,4, C. Allende Prieto5, D. Boyd6, J. Casares1, D. Engels3,
O. Giannakis7, E. T. Harlaftis8, J. Kube9, H. Lehto10,11, I. G. Mart́ınez-Pais1,12,
R. Schwarz13, W. Skidmore14, A. Staude13 and M. A. P. Torres15
1Instituto de Astrof́ısica de Canarias, Vı́a Láctea s/n, La Laguna, E-38205, Santa Cruz de Tenerife, Spain
2Department of Physics, University of Warwick, Coventry CV4 7AL, UK
3Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
4Department of Physics, Faculty of Science, Naresuan University, Phitsanulok, 65000, Thailand
5McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712, USA
6British Astronomical Association, Variable Star Section, West Challow, OX12 9TX, England, UK
7Institute of Astronomy and Astrophysics, National Observatory of Athens, P.O. Box 20048, Athens 11810, Greece
8Institute of Space Applications and Remote Sensing, National Observatory of Athens, PO Box 20048, Athens 11810, Greece
9Alfred-Wegener-Institut für Polar- und Meeresforschung, Bürgermeister-Smidt-Straße 20, 27568 Bremerhaven, Germany
10Tuorla Observatory, University of Turku, FIN-21500 Piikkiö, Finland
11Department of Physics, FIN-20014 University of Turku, Finland
12Departamento de Astrof́ısica, Universidad de La Laguna, Tenerife, Spain
13Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
14California Institute of Technology, Mail Code 105-24, Pasadena, CA 91125-24, USA
15Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
Accepted 2007. Received 2007
ABSTRACT
We present time-series optical photometry of five new CVs identified by the Ham-
burg Quasar Survey. The deep eclipses observed in HS 0129+2933 (= TT Tri),
HS 0220+0603, and HS 0455+8315 provided very accurate orbital periods of
3.35129827(65), 3.58098501(34), and 3.56937674(26) h, respectively. HS 0805+3822
shows grazing eclipses and has a likely orbital period of 3.2169(2) h. Time-resolved
optical spectroscopy of the new CVs (with the exception of HS 0805+3822) is also pre-
sented. Radial velocity studies of the Balmer emission lines provided an orbital period
of 3.55 h for HS 1813+6122, which allowed us to identify the observed photometric
signal at 3.39 h as a negative superhump wave. The spectroscopic behaviour exhibited
by all the systems clearly identifies them as new SW Sextantis stars. HS 0220+0603
shows unusual N ii and Si ii emission lines suggesting that the donor star may have
experienced nuclear evolution via the CNO cycle.
These five new additions to the class increase the number of known SW Sex
stars to 35. Almost 40 per cent of the total SW Sex population do not show eclipses,
invalidating the requirement of eclipses as a defining characteristic of the class and the
models based on a high orbital inclination geometry alone. On the other hand, as more
SW Sex stars are identified, the predominance of orbital periods in the narrow 3–4.5
h range is becoming more pronounced. In fact, almost half the CVs which populate
the 3–4.5 h period interval are definite members of the class. The dominance of SW
Sex stars is even stronger in the 2–3 h period gap, where they make up 55 per cent of
all known gap CVs. These statistics are confirmed by our results from the Hamburg
Quasar Survey CVs. Remarkably, 54 per cent of the Hamburg nova-like variables
have been identified as SW Sex stars with orbital periods in the 3–4.5 h range. The
observation of this pile-up of systems close to the upper boundary of the period gap
is difficult to reconcile with the standard theory of CV evolution, as the SW Sex stars
are believed to have the highest mass transfer rates among CVs.
Finally, we review the full range of common properties that the SW Sex stars
exhibit. Only a comprehensive study of this rich phenomenology will prompt to a full
understanding of the phenomenon and its impact on the evolution of CVs and the
accretion processes in compact binaries in general.
Key words: accretion, accretion discs – binaries: close – stars: individual: HS
0129+2933, HS 0220+0603, HS 0455+8315, HS 0805+3822, HS 1813+6122 – novae,
cataclysmic variables
⋆ E-mail:prguez@iac.es
c© 2007 RAS
http://arxiv.org/abs/0704.1129v2
2 P. Rodŕıguez-Gil et al.
1 INTRODUCTION
The Palomar-Green (PG) survey (Green, Schmidt &
Liebert 1986) led to the identification of several rela-
tively bright (V ∼ 15), deeply eclipsing cataclysmic vari-
ables (CVs) with orbital periods in the range 3 − 4 h,
namely SWSex (Penning et al. 1984), DWUMa (Shafter,
Hessman & Zhang 1988), BHLyn (Thorstensen et al. 1991;
Dhillon et al. 1992), and PXAnd (Thorstensen et al. 1991).
Szkody & Piché (1990) and Thorstensen et al. (1991) estab-
lished a number of common traits among these systems,
including unusually “V”-shaped eclipse profiles, the pres-
ence of He II λ4686 emission, a substantial orbital phase
lag (∼ 0.2 cycle) of the radial velocities of the Balmer
lines with respect to the motion of the white dwarf, and
single-peaked emission lines that display central absorp-
tion dips around orbital phases ≃ 0.4 − 0.7. The “SW
Sex phenomenon” was later extended to lower orbital in-
clinations after the identification of several grazingly eclips-
ing and non-eclipsing CVs which exhibit the spectroscopic
properties characteristic of the SW Sex class (WXAri:
Beuermann et al. 1992; Rodŕıguez-Gil et al. 2000; V795Her:
Casares et al. 1996; LSPeg: Mart́ınez-Pais, Rodŕıguez-Gil
& Casares 1999; Taylor, Thorstensen & Patterson 1999;
V442Oph: Hoard, Thorstensen & Szkody 2000).
The observational characteristics of the SWSex stars
are not easily reconciled with the properties of a sim-
ple, steady-state hot optically thick accretion disc which
is expected to be found in intrinsically bright, weakly-
magnetic CVs above the period gap. Nevertheless, a
variety of mechanisms have been invoked to explain
the behaviour of the SWSex stars, such as stream
overflow (Hellier & Robinson 1994; Hellier 1996), mag-
netic white dwarfs (Williams 1989; Casares et al. 1996;
Rodŕıguez-Gil et al. 2001; Hameury & Lasota 2002), mag-
netic propellers in the inner disc (Horne 1999), and self-
obscuration of the inner disc (Knigge et al. 2000). While
no unambiguous model for their accretion geometry has
been found so far, it is becoming increasingly clear that the
SWSex stars are not rare and unusual systems, but rep-
resent an important, if not dominant fraction of all CVs
in the orbital period range 3 − 4 h (Rodŕıguez-Gil 2005;
Aungwerojwit et al. 2005). Hence, a thorough investigation
of this class of systems is important in the context of under-
standing CV evolution as a whole.
In this paper, we present five new CVs identified in the
Hamburg Quasar Survey (HQS; Hagen et al. 1995) which
are classified as new SWSex stars on the basis of our follow-
up photometry and spectroscopy. The new discoveries bring
the total number of confirmed SWSex stars to 35, and we
discuss the global properties of these objects as a class.
2 OBSERVATIONS AND DATA REDUCTION
2.1 Identification
The five new CVs (Fig. 1) were selected for follow-
up observations upon the detection of emission lines in
their HQS spectra. Identification spectra of HS 0220+0603,
HS 0805+3822, and HS1813+6122 were respectively ob-
tained in October 1990 with the Calar Alto 3.5-m telescope,
in December 1996 with the 1.5-m Tillinghast telescope at
Table 2. Comparison stars used for the differential CCD pho-
tometry (see Fig. 1).
ID USNO-A2.0 R B
C01 1125-00509642 13.3 14.0
C02 1125-00510117 14.8 15.2
C03 0900-00553937 15.8 17.7
C04 0900-00554015 16.4 18.9
C05 1725-00217755 13.8 15.1
C06 1725-00218117 15.4 15.9
C07 1275-07186372 15.0 15.0
C08 1275-07186091 15.7 17.2
C09 1500-06459684 14.6 15.1
C10 1500-06458763 15.8 16.5
Fred Lawrence Whipple Observatory, and in June 1991 with
the Calar Alto 2.2-m telescope as part of QSO and galaxy
searches. HS 0129+2933 and HS0455+8315 were both iden-
tified as CVs in September 2000 using the Calar Alto 2.2-m
telescope as part of a dedicated search for CVs in the HQS
(Gänsicke et al. 2002).
HS 0129+2933 (= TT Tri) was already identified as
an eclipsing star by Romano (1978). At the time of writ-
ing this paper we were aware of a multicolour photometric
study by Warren, Shafter & Reed (2006), in which an eclipse
ephemeris is derived for the first time.
HS 0805+3822 was independently found as a CV in
the Sloan Digital Sky Survey (SDSSJ080908.39+381406.2,
Szkody et al. 2003) and was identified as a SWSex star.
Flux-calibrated, low resolution identification spectra of
the five new CVs are shown in Fig. 2. The spectra are dom-
inated by strong, single-peaked Balmer and He i emission
lines on top of blue continua. The high excitation emission
lines of He ii λ4686 and the C iii/N iii Bowen blend near
4640 Å are also observed, and are especially intense in HS
0455+8315. Table 1 summarises some properties of the new
2.2 Optical photometry
We obtained differential CCD photometry of the five new
CVs at seven different telescopes: the 2.2-m telescope at
Calar Alto Observatory, the 1.2-m telescope at Kryoneri Ob-
servatory, the 0.82-m IAC80 and the 1-m Optical Ground
Station at Observatorio del Teide, the 0.7-m telescope of
the Astrophysikalisches Institut Potsdam, the 1.2-m tele-
scope at Fred Lawrence Whipple Observatory, and the 0.7-
m Schmidt-Väisälä telescope at Tuorla Observatory, and the
0.35-m telescope at West Challow Observatory. Details on
the instrumentation are given in the notes to Table 3, which
also contains the log of the photometric observations. The
data obtained at Calar Alto, Kryoneri, and Tuorla were re-
duced with the pipeline described by Gänsicke et al. (2004),
which applies bias and flat-field corrections in MIDAS and
then uses Sextractor (Bertin & Arnouts 1996) to extract
aperture photometry for all objects in the field of view. The
reduction of the AIP data was carried out completely us-
c© 2007 RAS, MNRAS 000,
The SW Sextantis stars 3
HS0220+0603
HS0805+3822
E HS1813+6122
Figure 1. 10′ × 10′ finding charts for HS 0129+2933, HS 0220+0603, HS 0455+8315, HS 0805+3822, and HS 1813+6122 obtained from
the Digitized Sky Survey. See Table 2 for details on the comparison stars C1–C10.
Table 1. Properties of the five new SWSex stars. The coordinates and the B and R magnitudes were taken from the USNO-B
catalog (Monet et al. 2003). The J , H, and Ks magnitudes were taken from the 2MASS catalogue. The emission line parameters
are measured from the public SDSS spectrum.
HS0129+2933 HS 0220+0603 HS 0455+8315 HS 0805+3822 HS 1813+6122
Right ascension (J2000) 01h31m59.86s 02h23m01.65s 05h06m48.27s 08h09m08.40s 18h14m29.83s
Declination (J2000) +29◦49′22.3′′ +06◦16′50.0′′ +83◦19′23.3′′ +38◦14′06.5′′ +61◦23′34.5′′
Orbital period (h) 3.35129827(65) 3.58098501(34) 3.56937674(26) ≃ 3.22 ≃ 3.38
B/R magnitudes 15.3 / 14.7 16.5 / 16.2 15.2 / 14.6 14.7 / 14.6 15.1 / 14.5
J/H/Ks magnitudes 14.6 / 14.4 / 14.3 15.4 / 15.1 / 14.8 14.4 / 14.3 / 14.1 15.3 / 15.2 / 14.9 14.8 / 14.7 / 14.6
Hα EW [Å] / FHWM [km s−1] 61 / 1160 73 / 1390 50 / 1180 25 / 1040 22 / 1050
Hβ EW [Å] / FHWM [kms−1] 25 / 1265 43 / 1560 29 / 1250 11 / 970 6 / 1020
Hγ EW [Å] / FHWM [km s−1] 16 / 1350 25 / 1545 31 / 1540 10 / 1010 5 / 930
He I λ5876 EW [Å] / FHWM [kms−1] 8 / 1120 15 / 1270 – / – 2 / 720 1 / 720
He I λ6678 EW [Å] / FHWM [kms−1] 5 / 1090 9 / 1240 10 / 1240 4 / 830 – / –
He II λ4686 EW [Å] / FHWM [kms−1] – / – 6 / 2190 17 / 1900 4 / 700 – / –
ing MIDAS. IRAF1 was used to correct the OGS, IAC80, and
FLWO data for the bias level and flat-field variations, and
to compute Point Spread Function (PSF) magnitudes of the
target and comparison stars. Fig. 1 shows finding charts for
the five new CVs and indicates the comparison stars used
for differential photometry. The USNO R and B magnitudes
IRAF is distributued by the National Optical Astronomy Ob-
servatories.
of the comparison stars are given in Table 2. Sample light
curves are shown in Fig. 4. HS 0129+2933, HS 0220+0603,
and HS0455+8315 are deeply eclipsing, HS 0805+3822 dis-
plays evidence for grazing eclipses (similar to WXAri,
Rodŕıguez-Gil et al. 2000), and all five systems show sub-
stantial short time-scale variability. From the scatter in the
comparison star light curves we estimate that the differential
photometry is accurate to 1 per cent.
c© 2007 RAS, MNRAS 000,
4 P. Rodŕıguez-Gil et al.
Figure 2. Flux-calibrated optical spectra of the five new CVs.
2.3 Optical spectroscopy
The spectroscopic data were obtained with five different tele-
scopes: the 2.5-m Isaac Newton Telescope (INT) and the
2.56-m Nordic Optical Telescope (NOT) on La Palma, the
2.2-m and the 3.5-m telescopes at Calar Alto Observatory,
and the 2.7-m telescope at McDonald Observatory. The log
of spectroscopic observations can also be found in Table 3.
Details on the different telescope/spectrograph setups are
given in what follows:
1. At the INT we used the Intermediate Dispersion Spectro-
graph (IDS) with the R632V grating, the 2048× 4100 pixel
EEV10a CCD detector, and a 1.1′′ slit width. With this
setup we sampled the wavelength region λλ4400− 6700 at a
resolution (full width at half maximum, hereafter FWHM)
of ∼ 2.5 Å.
2. The Andalućıa Faint Object Spectrograph and Camera
(ALFOSC) was in place at the NOT. The spectra were im-
aged on the 2048×2048 pixel EEV chip (CCD #8). A spec-
tral resolution of ∼ 3.7 Å (FHWM) was achieved by using
the grism #7 (plus the second-order blocking filter WG345)
and a 1′′ slit width. The useful wavelength interval this con-
figuration provides is λλ3800− 6800.
3. The spectroscopy at the 2.2-m Calar Alto telescope was
performed with CAFOS. A 1.2′′ slit width and the G–100
grism granted access to the λλ4200 − 8300 range with a
resolution of ∼ 4.5 Å (FWHM) on the standard SITe CCD
(2048× 2048 pixel).
4. The double-armed TWIN spectrograph was used to carry
out the observations at the 3.5-m telescope in Calar Alto.
The blue arm was equipped with the T05 grating, while the
T06 grating was in place in the red arm. The wavelength
ranges λλ4070 − 5160 and λλ6080 − 7180 were sampled at
1.32 Å and 1.23 Å resolution (FWHM; 1.5′′ slit width) in
the blue and the red, respectively.
5. The Large Cassegrain Spectrometer (LCS) on the 2.7-m
telescope at McDonald Observatory was equipped with grat-
ing #43 and the TI1 800× 800 pixel CCD detector. The use
of a 1.0′′ slit width resulted in a resolution of 3 Å (FWHM)
and a wavelength range of λλ3800− 5030.
After the effects of bias and flat field structure were
removed from the raw images, the sky background was
subtracted. The one-dimensional target spectra were then
obtained using the optimal extraction algorithm of Horne
(1986). For wavelength calibration, a low-order polynomial
was fitted to the arc data, the rms being always smaller than
one tenth of the dispersion in all cases. The pixel-wavelength
correspondence for each target spectrum was obtained by
interpolating between the two nearest arc spectra. These re-
duction steps were performed with the standard packages
for long-slit spectra within IRAF.
2.4 HST/STIS far-ultraviolet spectroscopy
A single far-ultraviolet (FUV) spectrum of HS 0455+8315
was obtained with the Hubble Space Telescope/Space Tele-
scope Imaging Spectrograph (HST/STIS) as part of a large
survey of the FUV emission of CVs (Gänsicke et al. 2003).
The data were obtained using the G140L grating and the
52′′ × 0.2′′ aperture, resulting in a spectral resolution of
λ/∆λ ≃ 1000 and a wavelength coverage of λλ1150 − 1710.
The spectrum (Fig. 12) was obtained at an orbital phase
of ≃ 0.75, well outside the eclipse (see Sect. 4.5 for a dis-
cussion). The STIS acquisition image showed the system at
c© 2007 RAS, MNRAS 000,
The SW Sextantis stars 5
Table 3. Log of the observations.
Date UT Telescope Filter/ Exp. Frames
Grism (s)
HS0129+2933
2002 Aug 29 03:22-04:35 INT R632V 600 7
2002 Aug 31 05:00-05:40 INT R632V 600 4
2002 Sep 02 04:20-05:01 INT R632V 600 4
2002 Sep 03 04:39-05:20 INT R632V 600 4
2003 Dec 17 19:33-01:11 NOT Grism #7 600 27
2002 Sep 22 23:42-03:35 KY R 10 1035
2003 Sep 29 05:29-06:00 IAC80 clear 15 75
2003 Dec 15 18:24-23:56 CA22 clear 15 641
2003 Dec 16 18:07-19:35 CA22 clear 20 151
2003 Dec 25 20:06-21:52 CA22 clear 15 176
2006 Nov 21 22:22-23:39 WCO clear 60 73
2006 Dec 11 18:49-19:11 WCO clear 60 22
2006 Dec 16 22:21-23:38 WCO clear 60 73
HS0220+0603
2002 Oct 15 23:13-02:47 KY R 25 328
2002 Oct 17 01:13-03:44 KY R 25 279
2002 Dec 08 23:27-00:13 CA22 G-200 600 4
2002 Dec 16 18:25-19:20 CA22 G-200 600 5
2002 Dec 29 20:39-23:41 CA22 V 30 217
2002 Dec 31 18:30-21:02 CA22 V 30 145
2003 Jan 26 19:41-23:01 IAC80 V,R 100 119
2003 Jan 28 19:24-23:24 IAC80 V 120 102
2003 Jul 11 04:19-05:10 OGS clear 50 54
2003 Jul 13 04:08-05:25 OGS clear 30 119
2003 Sep 22 03:07-04:16 IAC80 clear 30 100
2003 Sep 29 00:07-01:24 IAC80 clear 30 113
2003 Nov 09 00:50-02:03 IAC80 R 60 58
2003 Nov 17 23:41-00:27 OGS clear 17 108
2003 Dec 15 19:28-21:04 NOT Grism #7 600 9
2003 Dec 16 19:26-02:33 NOT Grism #7 600 38
2006 Nov 21 19:15-20:11 WCO clear 60 55
2006 Dec 11 19:16-19:49 WCO clear 60 33
2006 Dec 11 22:47-23:23 WCO clear 60 36
2006 Dec 16 21:04-22:16 WCO clear 60 70
HS0455+8315
2000 Oct 20 18:57-21:44 AIP R 60 142
2000 Nov 10 17:30-21:48 AIP R 30 364
2000 Nov 16 16:45-21:14 AIP R 30 425
2000 Dec 04 17:09-17:50 AIP R,B 30 64
2000 Dec 05 16:39-18:03 AIP R,B 30 135
2000 Jan 01 03:30-05:31 CA35 T05/T06 600 11
2001 Jan 01 23:03-23:45 CA35 T05/T06 300 5
2001 Jan 02 00:32-04:38 CA35 T05/T06 400 33
2002 Dec 09 09:06 HST G140L 730 1
2006 Nov 21 20:30-22:02 WCO clear 60 88
2006 Nov 23 23:05-00:09 WCO clear 60 62
2006 Dec 08 20:05-21:03 WCO clear 50 68
Date UT Telescope Filter/ Exp. Frames
Grism (s)
2007 Jan 11 21:28-22:40 WCO clear 60 70
2007 Jan 13 23:33-00:15 WCO clear 50 50
2007 Jan 14 21:01-21:52 WCO clear 50 60
HS0805+3822
2005 Feb 11 19:28-04:32 Tuorla clear 90 325
2005 Feb 15 18:36-01:03 Tuorla clear 80 257
2005 Feb 28 04:39-10:00 FLWO1.2 clear 15-20 769
2005 Mar 01 03:12-09:56 FLWO1.2 clear 15 784
2005 Mar 04 05:59-09:33 FLWO1.2 clear 15-25 402
2005 Mar 24 20:03-00:04 Tuorla clear 30 373
2005 Mar 25 20:00-02:21 Tuorla clear 30 608
2005 Mar 27 03:07-08:18 FLWO1.2 clear 15-20 674
2005 Mar 28 04:06-07:12 FLWO1.2 clear 15-20 390
2005 Mar 29 04:48-08:04 FLWO1.2 clear 15-30 321
HS1813+6122
2000 Sep 24 20:23-20:54 CA22 R-100 600 2
2001 Aug 23 21:46-01:26 AIP R 30 346
2001 Aug 23 20:10-01:05 AIP R 60 251
2001 Aug 23 19:58-00:20 AIP R 60 199
2002 Jul 03 23:47-02:29 KY R 120 67
2002 Jul 05 19:54-22:31 KY R 120 69
2002 Aug 22 18:44-21:36 KY clear 10 660
2002 Aug 23 18:59-22:21 KY clear 10 800
2002 Sep 04 21:24-23:58 KY R 10 516
2002 Sep 04 00:23-00:54 INT R632V 600 4
2002 Sep 06 18:34-21:57 KY R 35 276
2003 Aug 20 18:11-21:43 KY clear 10 649
2002 Aug 27 22:38-00:11 INT R632V 600 9
2002 Aug 31 23:37-00:28 INT R632V 600 5
2002 Sep 01 21:55-22:37 INT R632V 600 4
2002 Sep 03 20:26-21:07 INT R632V 600 4
2003 Jun 28 22:49-00:35 KY R 60 93
2003 Jul 15 02:29-05:09 OGS clear 15 442
2003 Sep 23 20:23-00:05 IAC80 clear 7 690
2003 Sep 24 20:02-22:50 IAC80 clear 7 502
2003 Sep 25 21:18-23:48 IAC80 clear 10 400
2003 Sep 26 19:53-21:08 IAC80 clear 7 248
2003 Sep 27 19:36-23:19 IAC80 clear 7 680
2003 May 18 01:00-04:08 INT R632V 600 6
2003 May 19 02:48-04:13 INT R632V 600 8
2004 May 24 02:13-03:36 IAC80 clear 15 134
2004 May 25 01:44-05:19 IAC80 clear 15 411
2004 May 26 00:34-05:15 IAC80 clear 15 720
2003 Jun 29 00:52-03:58 CA22 G–100 600 15
2003 Jun 30 03:13-04:11 CA22 G–100 600 5
2004 Jul 17 04:17-04:50 McD #43 600 3
2004 Jul 18 04:11-05:19 McD #43 600 6
Notes on the instrumentation used for CCD photometry. CA22: 2.2-m telescope at Calar Alto Observatory, using CAFOS with
a 2k × 2k pixel SITe CCD; KY: 1.2-m telescope at Kryoneri Observatory, using a Photometrics SI-502 516 × 516 pixel CCD
camera; IAC80: 0.82-m telescope at Observatorio del Teide, equipped with Thomson 1k × 1k pixel CCD camera; OGS: 1-m
Optical Ground Station at Observatorio del Teide, equipped with Thomson 1k × 1k pixel CCD camera; AIP: 0.7-m telescope
of the Astrophysikalisches Institut Potsdam, with 1k × 1k pixel SITe CCD; FLWO: 1.2-m telescope at Fred Lawrence Whipple
Observatory, equipped with MINICAM containing three 2048 × 4608 EEV CCDs; Tuorla: 0.7-m Schmidt-Väisälä telescope at
Tuorla Observatory, equipped with a SBIG ST–8 CCD camera; WCO: 0.35-m telescope at West Challow Observatory with
SXV-H9 CCD camera.
c© 2007 RAS, MNRAS 000,
6 P. Rodŕıguez-Gil et al.
Table 4. Eclipse timings (given in HJD − 2450000), cycle num-
ber, and the difference in observed minus computed eclipse times
using the ephemerides in Eqs. (1–4). See Warren et al. (2006) for
additional eclipse timings of HS 0129+2933.
T0 Cycle O − C (s) T0 Cycle O − C (s)
HS0129+2933 2961.51100 2667 –1
2540.53210 0 –30 4061.32113 10038 10
2989.32739 3214 22 4081.31481 10172 –4
2989.46702 3215 21 4081.46397 10173 –8
2990.30418 3221 –36 4086.38789 10206 –2
2999.38161 3286 50 HS0455+8315
4061.46337 10892 5 1859.24683 0 –7
4081.29206 11034 20 1859.39549 1 –13
4086.45840 11071 –1 1865.34449 41 –12
HS0220+0603 1884.23301 168 8
2563.57452 0 42 1884.23333 168 24
2564.61852 7 2 4061.40138 14807 –28
2638.47610 502 –18 4063.48339 14821 –13
2666.37811 689 –3 4078.35639 14921 43
2666.37818 689 3 4112.41340 15150 –25
2668.31751 702 –29 4114.49583 15164 0
2668.46682 703 –20 4115.38843 15170 22
2831.70004 1797 –21 HS0805+3822
2834.68439 1817 –5 3413.42628 0 –9
2904.66298 2286 10 3455.38312 313 231
2911.52646 2332 4 3455.51361 314 –75
2952.55900 2607 40 3457.79144 331 –147
an approximate Rc magnitude of 15.4, close to the normal
high-state brightness.
3 PHOTOMETRIC PERIODS
3.1 HS 0129+2933, HS 0220+0603, and
HS 0455+8315
The deep eclipses detected in the light curves of
HS 0129+2933, HS 0220+0603, and HS0455+8315 provide
accurate information on the orbital periods of the systems.
Determining the time of mid-eclipse2 in SWSex stars is
notoriously difficult due to the asymmetric shape of the
eclipse profiles. We have therefore employed the following
method: the observed eclipse profile is mirrored in time
around an estimate of the eclipse centre, and the mirrored
profile is overplotted on the original eclipse data. The time
of mid-eclipse is then varied until the central part of both
eclipse profiles overlap as closely as possible. This empiri-
cal method proved to be somewhat more robust compared
to e.g. fitting a parabola to the eclipse profile. The mid-
eclipse times are reported in Table 4. An initial estimate of
the cycle count was then obtained by fitting eclipse phases
(φobserved0 −φ
−2 over a wide range of trial periods (Fig. 3).
Once an unambiguous cycle count was established, a lin-
ear eclipse ephemeris was fitted to the times of mid-eclipse.
For HS 0129+2933, we also included the 22 eclipse timings
2 The time of mid-eclipse, T0, is the time of inferior conjunction
of the donor star.
reported by Warren et al. (2006) in our calculations. The
resulting ephemerides are:
T0(HJD) = 2 452 540.53244(21) + 0.139637428(27) × E (1)
for HS 0129+2933, i.e. Porb = 3.35129827(65)) h,
T0(HJD) = 2 452 563.574036(73) + 0.149207709(14) × E (2)
for HS 0220+0603, i.e. Porb = 3.58098501(34) h, and
T0(HJD) = 2 451 859.24463(12) + 0.148724030(11) × E (3)
for HS 0455+8315, i.e. Porb = 3.56937674(26) h.
3.2 HS 0805+3822
Some of the light curves of HS 0805+3822 contain broad
dips that we interpret as grazing eclipses, similar to those
detected in WXAri (Rodŕıguez-Gil et al. 2000). The system
displays a varying level of short time-scale variability, and
we restrict the identification of the (presumed) eclipses to
the nights with low flickering activity (Fig. 4, Table 4). No
unique cycle count can be determined from the four eclipses
detected (Fig. 3), and the eclipse ephemerides determined
for the two most likely cycle count aliases are
T0(HJD) = 2453413.4264(22) + 0.1267611(75) × E , (4)
i.e. Porb = 3.0423(2) h, and
T0(HJD) = 2453413.4264(23) + 0.1340385(84) × E , (5)
i.e. Porb = 3.2169(2) h.
The data of the nights during which grazing eclipses were
detected folded over both periods are shown in Fig. 5.
As an alternative approach, we have subjected the
combined data from all nights to an analysis-of-variance
(after subtracting the nightly mean magnitudes) us-
ing Schwarzenberg-Czerny’s (1996) method, and find the
strongest signal at 3.21 h (Fig. 3). Folding all data on that
period gives a light curve which broadly resembles the
“eclipse” light curves mentioned above. We conclude that
the orbital period of HS 0805+3822 is ≃ 3.22 h.
3.3 HS 1813+6122
The light curve of HS 1813+6122 is characterized by rapid
(10− 20min) oscillations with a typical amplitude of 0.1 −
0.2mag, superimposed by modulations with time-scales of
hours and amplitudes of ≃ 0.1mag. We combined all data
after subtracting the nightly means, and calculated a Scargle
(1982) periodogram (Fig. 3, bottom panel). Two broad clus-
ters of signals are found at 3.38 and 3.54 h, with the stronger
signal at the shorter period. The same double-cluster struc-
ture repeats with lower amplitudes at several ±1 cycle/day
aliases. Fig. 6 shows the photometric data folded on either
period, after subtracting a sine fit with the respective other
period.
Many of the known SWSex stars display posi-
tive and/or negative superhumps in their light curves
(e.g. Patterson & Skillman 1994; Rolfe, Haswell & Pat-
terson 2000; Stanishev et al. 2002; Patterson et al. 2002;
Patterson et al. 2005), and the pattern observed in
c© 2007 RAS, MNRAS 000,
The SW Sextantis stars 7
Figure 3. Time series analysis of the CCD photometry of the
five new SWSex stars. The four top panels show the results of
eclipse-phase fits for the deeply eclipsing systems HS 0129+2933,
HS 0220+0603, and HS 0455+8315, as well as for the graz-
ing eclipser HS0805+3822. For HS 0805+3822, an AOV peri-
odogram (Schwarzenberg-Czerny 1996) is shown in the second-
lowest panel, and a Scargle (1982) periodogram is shown for
HS 1813+6122 in the bottom panel, with the likely orbital pe-
riod and superhump period indicated.
HS1813+6122 fits into that picture. On the basis of pho-
tometry alone, it is usually not possible to unambiguously
identify orbital and superhump periods, but based on the ra-
dial velocity study described below in Sect. 4.3, we suggest
that PSH = 3.38 h and Porb = 3.54 h.
4 SPECTROSCOPIC ANALYSIS
4.1 An overabundance of nitrogen in
HS 0220+0603
HS0220+0603 shows emission lines not usually seen in CVs.
In Fig. 7 we plot the average spectrum covering the region
λλ4870−6500. We identify a group of lines around λ5250 as
Fe ii transitions. They very likely are emission profiles plus
an absorption component since a deep absorption trough is
observed at the position of Fe ii λ5169. The He i λ5016 line
is abnormally broad and has a strange profile with three
peaks. This is probably the effect of blended He i and Fe ii
emission (at λ5018). But the most notorious feature is the
Figure 5. Phase-folded light curves of HS 0805+3822. Top and
middle panels: data from the nights with low flickering activity
folded over the eclipse ephemerides given by Eqs. (4,5). Bottom
panel : all data folded over the period determined from an analysis-
of-variance periodogram (ORT, Schwarzenberg-Czerny 1996).
Figure 6. Phase-folded light curves of HS 1813+6122. Top panel :
all data folded over PSH = 3.38 h, the strongest signal detected in
the periodogram (Fig. 3), after subtracting a sine fit with Porb =
3.54 h, the period of the neighbouring signal. Bottom panel : all
data folded over Porb = 3.54 h after subtracting a sine fit with
PSH = 3.38 h. We interpret PSH and Porb as the superhump and
orbital periods, respectively.
c© 2007 RAS, MNRAS 000,
8 P. Rodŕıguez-Gil et al.
Figure 4. Sample light curves of the five new SWSex stars. From top to bottom: HS0129+2933, HS 0220+0603, HS0455+8315,
HS 0805+3822, and HS 1813+6122. HS 0805+3822 displays grazing eclipses. Those indicated by ticks above the light curves were used to
calculate an ephemeris.
broad N ii λ5680 emission. Another triple-peaked emission
line profile is found at λ6350 which we identify as Si ii emis-
sion. The N ii λ5680 transition has been observed in the in-
termediate polar HS 0943+1404 (Rodŕıguez-Gil et al. 2005),
and we interpret its presence as evidence of an anomalously
large abundance of nitrogen.
An observed overabundance of nitrogen in the accre-
tion flow is directly linked to the chemical abundances of
the donor star, and suggests that the donor has under-
gone nuclear evolution via the CNO cycle. This implies that
the donor had an initial mass of & 1.2M⊙ and the system
evolved through a phase of thermal-time scale mass trans-
fer (Schenker et al. 2002; Podsiadlowski, Han & Rappaport
2003). The theoretical models predict that up to 1/3 of all
CVs may have undergone nuclear evolution. This seems to
be confirmed by the substantial number of systems with
evolved donor stars that have been found (e.g. Jameson,
King & Sherrington 1980; Bonnet-Bidaud & Mouchet 1987;
Thorstensen et al. 2002a, 2002b; Gänsicke et al. 2003).
4.2 Radial velocities
Radial velocity curves of the Hα and He ii λ4686 emis-
sion lines were computed for the eclipsing systems, except
for HS 0129+2933 where He ii λ4686 is too weak and only
Hα was measured. The individual velocity points were ob-
tained by cross-correlating the individual profiles with single
Gaussian templates matching the FWHMs of the respective
average line profiles (see Table 1). Before measuring the ve-
locities, the normalised spectra were re-binned to a uniform
velocity scale centred on the rest wavelength of each line.
The radial velocity curves of the three deeply-eclipsing sys-
tems folded on their respective orbital periods are presented
in Fig. 8.
We fitted sinusoidal functions of the form:
Vr = γ −K sin [2π (ϕ− ϕ0)]
to the radial velocity curves. The fitting parameters are
shown in Table 5. Note that the γ and K parameters given
in the table are not the actual systemic velocity and radial
velocity amplitude of the white dwarf (K1). The Hα line
in HS 0129+2933, HS 0220+0603, and HS0455+8315 is de-
layed with respect to the motion of the white dwarf by ϕ0 ∼
c© 2007 RAS, MNRAS 000,
The SW Sextantis stars 9
Table 5. Radial velocity curves fitting parameters.
Line γ (km s−1) K (km s−1) ϕ0
HS0129+2933
Hα 69± 1 282± 1 0.240± 0.001
HS0220+0603
Hα −37.0± 0.7 186± 1 0.170± 0.001
He II λ4686 −88± 16 295 ± 22 0.24± 0.01
HS0455+8513
Hα −27± 1 238± 1 0.155± 0.001
He II λ4686 5± 1 125± 7 0.14± 0.01
Figure 7. Orbitally-averaged spectrum of HS 0220+0603. Notice
the unusual N ii λ5680 emission line.
0.2. The He ii λ4686 radial velocity curves of HS 0220+0603
and HS0455+8315 show the same phase offset. This phase
lag indicates that the main emission site is at an angle
(∼ 72◦) to the line of centres between the centre of mass of
the binary and the white dwarf. The radial velocity curves
are not sinusoidal in shape, showing significant distortion
mainly at phases 0 (mid-eclipse) and 0.5. The spikes at ϕ ∼ 0
are due to a rotational disturbance, caused by the fact that
the secondary first occults the disc material approaching us
and then the receding material. This translates into a red
velocity spike before mid-eclipse and a blue spike after it.
4.3 The orbital period of HS 1813+6122
Of the five new CVs, HS 1813+6122 is the only one not
showing eclipses, but the photometry suggests a possible or-
bital period of 3.54 h (Sect. 3.3). In an attempt to confirm
this value we performed a period analysis on the Hα and
Hβ radial velocities. The velocities were derived by using
the double Gaussian method of Schneider & Young (1980)
(which gave better results than the single Gaussian tech-
nique), adopting a Gaussian FWHM of 200 km s−1 and
a separation of 1600 km s−1. The 2002 September 1 and
2003 May INT data were obtained under poor observing
conditions and were therefore excluded from the analysis.
In Fig. 9 we show the resulting analysis-of-variance peri-
odogram computed from the combined Hα and Hβ radial
velocity curves. The periodogram shows its strongest peak
Figure 8. Radial velocity curves of the deeply-eclipsing sys-
tems HS0129+2933, HS0220+0603, and HS 0455+8315 folded on
their respective orbital periods after averaging the data into 20
phase bins. The Hα velocities are delayed by ∼ 0.1 − 0.2 or-
bital cycle with respect to the photometric ephemerides. The
same is observed for the He ii λ4686 lines in HS 0220+0603 and
HS 0455+8315. The orbital cycle has been plotted twice.
at a period of 3.53 h, which confirms the value given by the
photometric data as well as the presence of a negative su-
perhump in the light curve of HS 1813+6122. A sine fit to
the longest spectroscopic run (2003 Jun 29) with the pe-
riod fixed at the above value yielded a tentative zero-phase
time of T0(HJD) = 2 452 819.597(2). A preliminary trailed
spectrum revealed the characteristic SWSex high-velocity S-
wave reaching its maximum blue velocity at relative phase
∼ 0.3 (the phases are defined by using the above T0). By
analogy with the eclipsing systems presented in this paper
(Sect. 4.4), and with the eclipsing SW Sex stars in general
(see e.g. Hellier 1996; Hoard & Szkody 1997; Hellier 2000;
Rodŕıguez-Gil et al. 2001), this is expected to happen at ab-
solute phase ϕ ∼ 0.5. Therefore, the Balmer radial velocities
of HS 1813+6122 are delayed by ϕ0 ∼ 0.2 with respect to the
white dwarf motion, a defining characteristic of the SWSex
stars. Correcting for this we get a new time of zero phase
of T0(HJD) = 2 452 819.568(2). Fig. 9 shows the combined
Hα and Hβ velocities folded on the orbital period with the
new phase definition.
4.4 Trailed spectra
Trailed spectrograms of several emission lines of
HS 0129+2933, HS 0220+0603, and HS0455+8315 were
constructed after re-binning the spectra on to an uniform
velocity scale centred on the rest wavelength of each line.
They are shown in Fig. 10. The Hα and Hβ line emis-
sion is dominated by a high-velocity emission component
which neither follows the phasing of the primary nor the
secondary. These S-waves reach their bluest velocity at
ϕ ∼ 0.5 in the three eclipsing systems and have a velocity
amplitude of & 600 km s−1. Weaker emission can also be
seen underneath the dominant S-wave, possibly originating
c© 2007 RAS, MNRAS 000,
10 P. Rodŕıguez-Gil et al.
Figure 9. Top: Scargle periodogram of the Hα+Hβ radial veloc-
ity curves of HS 1813+6122. Bottom: phase folded radial veloc-
ity curve (no phase binning applied). See text for details on the
adopted T0. A full cycle has been repeated.
in the accretion disc. An absorption component is observed
moving across the lines from red to blue, reaching maximum
strength also at ϕ ∼ 0.5.
The He i lines also display this high-velocity S-wave with
the same phasing as the Balmer lines, as well as the absorp-
tion component. The He i λ4472 line in HS0220+0603 shows
it for approximately three quarters of an orbit, going well
below the continuum.
Only HS0220+0603 and HS 0455+8315 have a
He ii λ4686 line strong enough to produce a clear trailed
spectrogram. While in the former the emission seems to
be entirely dominated by the high-velocity component, two
components are observed in the latter: a low-velocity one
(the strongest), and a weaker component which is probably
the same S-wave we observe in the Balmer and He i lines.
No significant absorption is detected in He ii λ4686.
In Fig. 11 we present the Hα trailed spectrum of
HS 1813+6122 after averaging the individual spectra into
20 orbital phase bins. The line shows a high-velocity S-wave
with maximum blue velocity at ∼ −2000 km s−1.
4.5 The FUV spectrum of HS 0455+8315
The ultraviolet spectrum of HS 0455+8315 (obtained at
ϕ ≃ 0.75) displays very strong emission lines of He, C,
N, O, and Si, with line flux ratios compatible with those
observed in the majority of CVs (Mauche, Lee & Kall-
man 1997), indicating normal chemical abundances of the
donor star. The slope of the FUV continuum is nearly flat
as observed in a number of deeply eclipsing SWSex stars
(e.g. DWUMa, Szkody 1987; Knigge et al. 2004; PXAnd,
Thorstensen et al. 1991; BHLyn, Hoard & Szkody 1997), .
Figure 11. Hα trailed spectra of HS 1813+6122. Black represents
emission. The grey scale has been adjusted to enhance the high-
velocity S-wave (left) and the line core (right). A whole cycle has
been repeated.
It has been argued that a relatively cold structure shields the
inner disc and the white dwarf from view in high-inclination
SWSex stars during the high state, specifically supported by
the FUV detection of the hot white dwarf in DWUMa dur-
ing a low state (Knigge et al. 2000; Araujo-Betancor et al.
2003).
5 SWSEX MEMBERSHIP
The five new CVs presented in this paper have very much in
common. In their average spectra the Balmer and He i lines
display both single- or double-peaked profiles. The double-
peaked profiles are likely a consequence of phase-dependent
absorption components as the trailed spectra show. The
lines are also characterised by highly asymmetric profiles
with enhanced wings. The trailed spectra reveal the pres-
ence of a high-velocity emission S-wave in all the systems
with extended wings reaching a maximum velocity between
∼ ±2000 km s−1 (HS 1813+6122) and ∼ ±1000 km s−1 in
the eclipsing systems. The tendency of non-eclipsing SW Sex
stars to show broader S-waves may be evidence of emitting
material with a vertical velocity gradient such as a mass
outflow.
The radial velocity curves also show a distinctive SW
Sex feature. They are delayed with respect to the motion
of the white dwarf, so that the red-to-blue crossing takes
place at ϕ ∼ 0.2 instead of ϕ = 0. On the other hand, the
eclipsing systems also display a discontinuity around mid-
eclipse, probably a rotational disturbance, which indicates
that part of the line emission comes from the accretion disc.
All the features described above are defining charac-
teristics of the SW Sex stars (see Thorstensen et al. 1991
and Rodŕıguez-Gil, Schmidtobreick & Gänsicke 2007). Even
though each system exhibits its own peculiarities (e.g. un-
usual spectral lines in HS 0220+0603 and strong He ii λ4686
emission in HS0455+8315), they all share the characteris-
tic SWSex behaviour. We therefore classify them all as new
SWSex stars.
c© 2007 RAS, MNRAS 000,
The SW Sextantis stars 11
Figure 10. Trailed spectra of (from top to bottom) HS0129+2933, HS 0220+0603, and HS 0455+8513. The individual spectra were
averaged into 20 phase bins. Black represents emission. Unsampled phase bins are indicated by a blank spectrum. A full orbital cycle
has been repeated for continuity.
6 THE SW SEX STARS IN THE CONTEXT OF
CV EVOLUTION
6.1 How are the SW Sex stars discovered as CVs?
CVs are found by a number of means. Many of them dis-
play abrupt brightenings as a result of a dwarf nova eruption
or a nova explosion. They also show a rich variety of pho-
tometric variations like eclipses, orbital modulations, rapid
oscillations, ellipsoidal modulations, etc. Others have been
discovered by their blue colour, the emission of X-rays, or
the presence of strong emission lines in their optical spectra.
The total number of definite members of the SW Sex
class so far known amounts to 35, out of which a remarkable
number of 18 (51 per cent) has been found in UV-excess sur-
c© 2007 RAS, MNRAS 000,
12 P. Rodŕıguez-Gil et al.
Figure 12. HST/STIS far-ultraviolet spectrum of
HS 0455+8315.
veys (i.e. blue colour). This is not surprising, as the optical
spectra of the SWSex stars in the high-state is characterised
by a very blue continuum. On the other hand, 11 SWSex
stars (31 per cent) have been identified as CVs from their
emission line spectra, five of which were discovered in the
HQS. Only four and two SWSex stars have been found be-
cause of their brightness variability (including 3 novae) and
X-ray emission, respectively.
Any sort of CV search has its own selection effects, and
the classification of a CV as a SW Sex star is no excep-
tion. In fact, the deep eclipses that many of the SW Sex
stars show initially made the sample clearly biased towards
high inclination systems. This led many authors to link the
SW Sex phenomenon to a mere inclination effect. At the
last count, 13 out of a total of 35 SW Sex stars (37 per
cent) do not display eclipses and are bona fide members of
the class. Although an inclination effect may certainly be
important (see the case of HL Aqr in Rodŕıguez-Gil et al.
2007), the increasing number of non-eclipsing systems poses
serious difficulties to any model resting solely on a high or-
bital inclination.
In the following section we discuss on the impact of
SW Sex stars in the (spectroscopic) HQS sample, which is
unaffected by the high-inclination selection effect.
6.2 The role of the SW Sex stars in the big family
of nova-like CVs
In Table 6 we list the 35 known SW Sex stars along with their
orbital periods. Before doing any statistics we want to stress
the fact that Table 6 does not intend to present a definitive
census of the SW Sex stars. This is because their defining
characteristics are continuously evolving as we dig deeper
into the understanding of this class of CVs. For comparison
(and probably completeness) we also point the reader to Don
Hoard’s Big List of SW Sex stars.3
Table 6 shows that a significant 37 per cent of the fam-
ily does not show eclipses, confirming that the high incli-
nation requirement is merely a selection effect. The orbital
3 http://spider.ipac.caltech.edu/staff/hoard/biglist.html
Figure 13. Period distributions of confirmed SWSex stars (left
panel) and nova-like variables and classical novae that are not
known to exhibit SWSex behaviour. The orbital period gap and
the 3–4 h orbital period range are indicated in light and dark
grey, respectively.
period distribution of the known sample of SW Sex stars is
presented in the left panel of Fig. 13. The combined dis-
tributions for non-SW Sex nova-likes and nova remnants
are also plotted for comparison. The nova-likes and novae
were selected from the Ritter (Ritter & Kolb 2003), CVcat
(Kube et al. 2003), and Downes (Downes et al. 2005) cata-
logues. Only systems with a robust orbital period determi-
nation are included.
These orbital period distributions reveal important fea-
tures. About 40 per cent (35 out of 93) of the whole nova-
like/classical nova population (which is preferentially found
above the period gap; only 12 are below it) are indeed SW
Sex stars. Even more remarkable is the fact that the SW
Sex stars represent almost half (26 out of 53) the CVs in
the narrow 3−4.5 h orbital period range. Above 4.5 h things
radically change and only 14 per cent (4 out of 28) of the
nova-like/nova population are known to be SWSex stars.
The impact of SW Sex stars in the gap is also striking, as
55 per cent of all the nova-like/nova gap inhabitants are
members of the class.
The non-SW Sex nova-like/nova Porb distribution also
shows a significant fraction of systems in the 3 − 4.5 h in-
terval. This nicely depicts the tendency of nova-likes to ac-
cumulate in the 3− 4.5 h period range. Therefore, it is very
likely that more SW Sex stars are still to be found. In fact,
time-resolved spectroscopic studies like the one reported in
Rodŕıguez-Gil et al. (2007) are revealing the SWSex nature
of many previously poorly studied nova-likes in this range.
If the rate of detection and identification of SW Sex systems
remains high the dominance of this class at the upper edge
of the gap will eventually become even more pronounced.
It is possible, however, that these numbers are the re-
sult of a selection effect as the majority of SW Sex stars are
bright, making them easily accesible to observations. There-
fore, one can argue that a proper characterisation of the
whole population of CVs above the gap needs to be made
before addressing any conclusion. However, the fact the ma-
jority of SW Sex stars have orbital periods between 3 and
4.5 h is a well established fact.
6.2.1 The impact of SW Sex stars on the HQS CV sample
During the course of our spectroscopic search we have dis-
covered 53 new CVs in the area/magnitude range covered
c© 2007 RAS, MNRAS 000,
The SW Sextantis stars 13
Table 6. The SW Sex stars.
Object Porb (h) Eclipses References
V348Pup 2.44 Yes 1
V795Her 2.60 No 2
RXJ1643.7+3402 2.89 No 3
V442Oph 2.98 No 4
AHMen 3.05 No 5
HS 0728+6738 3.21 Yes 6
HS 0805+3822 3.21 Grazing 7,
this paper
SWSex 3.24 Yes 8
HLAqr 3.25 No 5
DWUMa 3.28 Yes 8
SDSS J132723.39+652854.2 3.28 Yes 9
WXAri 3.34 Grazing 10
HS 0129+2933 3.35 Yes This paper
V1315Aql 3.35 Yes 8
BOCet 3.36 No 5
AHPic 3.41 No 5
VZScl 3.47 Yes 11
LNUMa 3.47 No 5
RRPic 3.48 Grazing 12
PXAnd 3.51 Yes 8
V533Her 3.53 No 13
HS 1813+6122 3.54 No This paper
HS 0455+8315 3.57 Yes This paper
HS 0220+0603 3.58 Yes This paper
HS 0357+0614 3.59 No 14
V380Oph 3.70 No 5
BHLyn 3.74 Yes 15
UUAqr 3.93 Yes 16
LXSer 3.95 Yes 17
V1776Cyg 3.95 Yes 18
LSPeg 4.19 No 19
V347Pup 5.57 Yes 20
RWTri 5.57 Yes 21
V363Aur 7.71 Yes 22
BTMon 8.01 Yes 23
References: 1 Rodŕıguez-Gil et al. (2001); 2 Casares et al.
(1996); 3 Mickaelian et al. (2002); 4 Hoard et al. (2000); 5
Rodŕıguez-Gil et al. (2007); 6 Rodŕıguez-Gil et al. (2004); 7
Szkody et al. (2003); 8 Thorstensen et al. (1991); 9 Wolfe et al.
(2003); 10 Beuermann et al. (1992); 11 Moustakas & Schlegel
(1999); 12 Schmidtobreick, Tappert & Saviane (2003); 13
Rodŕıguez-Gil & Mart́ınez-Pais (2002); 14 Szkody et al. (2001);
15 Thorstensen, Davis & Ringwald (1991); 16 Hoard et al. (1998);
17 Young, Schneider & Shectman (1981); 18 Garnavich et al.
(1990); 19 Taylor et al. (1999); 20 Thoroughgood et al. (2005);
21 Groot, Rutten & van Paradijs (2004); 22 Thoroughgood et al.
(2004); 23 Smith, Dhillon & Marsh (1998)
by the HQS (≃ 13600 square degrees/17.5 . B . 18.5; see
Hagen et al. 1995). So far, we have determined the orbital
period for 43 of them in a huge observational effort. Our
preliminary results support the SW Sex excess within the
3 − 4.5 h range. Remarkably, 54 per cent (7 out of 13) of
all the newly-discovered HQS nova-likes are indeed SW Sex
stars, which is in agreement with the distribution discussed
above. This gives further strength to the significancy of the
observed (and still unexplained) pile-up of SW Sex stars in
the 3− 4.5 h region.
6.3 CV evolution and the SW Sex stars
This accumulation of systems just above the period gap seri-
ously challenges our current understanding of CV evolution.
The SW Sex stars are intrinsically very luminous, as the
brightness of systems like DW UMa indicates. DW UMa is a
SW Sex star likely located at a distance between ∼ 590−930
pc (Araujo-Betancor et al. 2003) that has an average mag-
nitude of V ∼ 14.5, even though it is viewed at an orbital
inclination of ∼ 82◦. Therefore, in order to show such high
luminosities, either the SW Sex stars have an average mass
transfer rate well above that of their CV cousins, or another
source of luminosity exists. Neither the Rappaport, Joss &
Verbunt (1983) nor the (empirical) Sills, Pinsonneault &
Terndrup (2000) angular momentum loss prescriptions ac-
count for a largely enhanced mass transfer rate (Ṁ) in the
period interval where most of the SW Sex stars reside. In
this regard, nuclear burning has been suggested as an extra
luminosity source (Honeycutt 2001), but the necessary con-
ditions for the burning to occur can only be found in the
base of a magnetic accretion funnel, suggesting a magnetic
nature (Honeycutt & Kafka 2004). Temporary cessation of
nuclear burning would in principle explain the VY Scl low
states that many SW Sex and other nova-likes undergo. If
this is true, the majority of CVs above the period gap have
to be magnetic, in stark contrast with a non-magnetic ma-
jority below the gap. However, the fact that some dwarf
novae (like HT Cas and RX And) also show low states (e.g.
Robertson & Honeycutt 1996; Schreiber, Gänsicke & Mat-
tei 2002) argues against this possibility, suggesting that the
VY Scl states are likely the product of a decrease in the
mass transfer rate from the donor star caused by starspots,
as already proposed by Livio & Pringle (1994). The obser-
vational results of Honeycutt & Kafka (2004) appear to sup-
port this hypothesis (see also e.g. Howell et al. 2000).
All of the above arguments point to accretion at a very
high Ṁ as the most likely cause for the high luminosity
observed in the SW Sex stars. One possibility could be en-
hanced mass transfer due to irradiation of the inner face
of the secondary star by a very hot white dwarf. In fact, a
number of nova-like CVs in the ∼ 3 − 4 h orbital period
range (including the SW Sex star DW UMa) have been ob-
served to harbour the hottest white dwarfs found in any
CV (see Araujo-Betancor et al. 2005), with effective tem-
peratures peaking at ∼ 50 000 K. These high temperatures
are most likely the result of accretion heating, as CVs are
thought to spend on average ∼ 2 Gyr as detached binary
systems (Schreiber & Gänsicke 2003), enough time for the
white dwarfs to cool down to . 8000 K. Hence, the high
effective temperatures measured in the 3 − 4 h range is
a measure of a very high secular mass accretion rate of
∼ 5 × 10−9 M⊙ yr
−1 (Townsley & Bildsten 2003), higher
than predicted by angular momentum loss due to magnetic
braking. Irradiation of the donor star has been observed in
the non-eclipsing SW Sex stars HL Aqr, BO Cet, and AH
Pic (Rodŕıguez-Gil et al. 2007), which supports the above
hypothesis. Alas, the question of why the SW Sex stars have
the highest mass transfer rates is still lacking a satisfactory
explanation in the context of the current CV evolution the-
c© 2007 RAS, MNRAS 000,
14 P. Rodŕıguez-Gil et al.
6.4 A rich phenomenology to explore
It is becoming apparent that the SW Sex phenomenon is
not restricted to the (mainly) spectroscopic properties ini-
tially introduced by Thorstensen et al. 1991. These maver-
ick CVs are now known to show a much more intricate be-
haviour. Therefore, only a comprehensive study of this rich
phenomenology will definitely lead to a full understanding of
the SW Sex stars. In what follows we will review the range of
common features exhibited by the SWSex stars and which
implications can be derived from them.
6.4.1 Superhumps
A significant one third of the SWSex stars is known
to show apsidal (positive) or/and nodal (negative) super-
humps, which are large-amplitude photometric waves mod-
ulated at a period slightly longer or slightly shorter than
the orbital period, respectively. Positive superhumps are be-
lieved to be the effect of an eccentric disc which is forced to
progradely precess by the tidal perturbation of the donor
star (Whitehurst 1988). On the other hand, negative su-
perhumps are likely linked to the retrograde precession of a
warped accretion disc (Murray et al. 2002). Vertical changes
in the structure of the disc may be triggered by the torque
exerted on the disc by the tilted, dipolar magnetic field of
the secondary star. Apparently, positive and negative super-
humps are independent and can either coexist or alternate
with a time scale of years.
The detection of superhumps in the SWSex stars is of
great importance as they exhibit the largest superhump pe-
riod excesses. Therefore, they are fundamental in calibrat-
ing the period excess–mass ratio relationship for CVs (see
Patterson et al. 2005).
6.4.2 Variable eclipse depth
The continuum light curves of some eclipsing members of the
class reveal that the eclipse depth varies with a time-scale
of several days. So far this variability has only been stud-
ied in PXAnd (Stanishev et al. 2002; Boffin et al. 2003) and
DWUMa (B́ıró 2000; Stanishev et al. 2004, not to be con-
fused with the changing eclipse depth in different brightness
states). In PX And, Stanishev et al. 2002 identify this long
periodicity with the precession period of the accretion disc,
which may also be true for DW UMa. The actual mechanism
is not yet understood, although the eclipses of a retrogradely
precessing, warped disc may account for what is observed in
PXAnd (Boffin et al. 2003).
6.4.3 Low states
Half of the nova-likes known to undergo VYScl faint states
are SWSex stars. During these events the system brightness
can drop by up to ∼ 4 − 5mag and can remain that low
for months. As the disc warping effect mentioned above,
these low states may be controlled by the strong magnetic
activity of the donor star, and are believed to be driven by
a sudden drop of the accretion rate from the secondary star
due to large starspots located in the area around the inner
Lagrangian point L1 (Livio & Pringle 1994).
Interestingly, the disced VYScl stars concentrate in the
3 − 4.5 h orbital period stripe as the SWSex stars do. It
would therefore not be a surprise to find that all nova-likes
in that range are actually VYScl stars and even SWSex
stars. Nevertheless, the presence of large starspots around
the L1 point still have to be observationally confirmed. Us-
ing Roche tomography techniques, Watson, Dhillon & Shah-
baz (2006) discovered a heavily spotted secondary star in
AEAqr (Porb = 9.88 h) with a spot distribution resembling
that of other rapidly rotating, low-mass field stars. A stellar
atmosphere plagued with spots has been also observed in the
pre–CV V471Tau (Porb = 12.51 h, Hussain et al. 2006). Un-
fortunately, the high spectral and time resolution required
to image the M-dwarf donor in a CV with Porb ≃ 3.5 h at
a distance of many hundred parsecs is currently beyond ob-
servational reach.
6.4.4 Quasi-periodic oscillations
On the grounds of short-term variability, the SWSex stars
are also characterised for exhibiting quasi-coherent mod-
ulations in their light curves. In a compilation of CVs
displaying rapid oscillations, Warner (2004) lists 9 (only
two deeply eclipsing) SWSex stars known to have quasi-
periodic oscillations (QPOs) with a predominant time-scale
of ∼ 1000 − 2000 s. For example, the non-eclipsing SWSex
stars V442Oph and RXJ1643.7+3402 show strong QPO sig-
nals dominating over other underlying higher-coherence os-
cillations (Patterson et al. 2002). These results are based on
hundreds of hours of photometric data, whose power spec-
tra showed rapid frequency changes of the main signals in
less than a day. Similar rapid variability was also detected
in the optical light curve of the SWSex star HS 0728+6738
(Rodŕıguez-Gil et al. 2004), with a prominent signal (coher-
ent for at least 20 cycles) at ∼ 600 s.
6.4.5 Emission-line flaring
In connection with the QPO activity, the fluxes and EWs
of the emission lines of some SWSex stars show modu-
lations at the same time-scales (a phenomenon known as
emission-line flaring): ∼ 1800 s in BTMon (Smith et al.
1998), ∼ 2000 s in LSPeg (Rodŕıguez-Gil et al. 2001), ∼
1400 s in V533Her (Rodŕıguez-Gil & Mart́ınez-Pais 2002),
∼ 1800 s in DWUMa (V. Dhillon, private communication),
∼ 2400 s in RXJ1643.7+3402 (Mart́ınez-Pais, de la Cruz
Rodŕıguez & Rodŕıguez-Gil 2007), and ∼ 1200 s in BOCet
(Rodŕıguez-Gil et al. 2007). Remarkably, the radial veloci-
ties measured in the last two objects are also modulated
at the flux/EW periodicities, which suggests that emission-
line flaring has to do with the dynamics of the line emit-
ting source, and is not due to e.g. random fluctuations in
the disc continuum emission. On the other hand, although
DW UMa does exhibit emission-line flaring in the optical,
such line variability was not detected in the far ultraviolet
(Hoard et al. 2003). Since similar flaring in the optical is
observed in the intermediate polar CVs (IPs; e.g. FOAqr
Marsh & Duck 1996) caused by the rotation of the mag-
netic white dwarf, all the described rapid variations seen
in many SWSex stars have been associated to the pres-
ence of magnetic white dwarfs (Rodŕıguez-Gil et al. 2001;
Patterson et al. 2002). Nevertheless, the far ultraviolet data
c© 2007 RAS, MNRAS 000,
The SW Sextantis stars 15
of DW UMa presented by Hoard et al. (2003) can also be ex-
plained with a stream overflow model, but do not necessary
exclude a magnetic scenario either.
6.4.6 Variable circular polarisation
Despite the fact that circular polarisation is not commonly
detected in the majority of IPs, it is a sine qua non condition
for IP membership (see further requirements in Patterson
1994). The cyclotron radiation emitted by the accretion
columns (built up by disc plasma forced by the magnetic
field to supersonically fall on to the white dwarf surface) is
known to be circularly polarised, thus the detection of a sig-
nificant level of circular polarisation is a unequivocal sign of
magnetic accretion. This, and the possible magnetic nature
of the QPO and line flaring activity prompted to the search
for circular polarisation in the SWSex stars.
Rodŕıguez-Gil et al. (2001) found circular polarisation
modulated at 1776 s with a peak-to-peak amplitude of 0.3
per cent in LSPeg. Remarkably, the flaring observed in the
Hβ high-velocity S-wave was modulated at 2010 s, which is
just the synodic period between the polarisation period and
the orbital period. V795Her also revealed variable circu-
lar polarisation with a periodicity of 1170 s (or twice that)
and showed an increasing polarisation level with wavelength
(Rodŕıguez-Gil et al. 2002) as is expected for cyclotron emis-
sion. In addition, RXJ1643.7+3402 shows circular polarisa-
tion modulated at 1163 s (Mart́ınez-Pais et al. 2007).
The 1776-s period of LSPeg was confirmed by Baskill,
Wheatley & Osborne (2005) who detected a coherent modu-
lation at 1854 s in ASCA X-ray light curves. The coincidence
of both periods indicates a common origin, with the X-ray
period reported by Baskill et al. likely being a more accurate
measurement of the white dwarf spin period.
Although polarimetric studies of many other SWSex
stars have to be done, the results obtained so far suggest
that magnetic accretion may play an important role in the
SWSex phenomenon. However, with the few such studies
so far at hand it is not possible to address any conclusion
regarding the impact of magnetism in the whole class.
Despite the broad implications in our understanding of
accretion that the study of the SWSex stars have, the quest
for a successful, global explanation of the phenomenon has
been unfruitful so far. This, in addition to the (yet unex-
plained) fact that the majority of SWSex stars (and many
nova-likes) largely populate the narrow orbital period stripe
between 3 and 4.5 hours, are seriously shaking the grounds
on which CV evolution theory stands. Further study of these
maverick systems will certainly provide fundamental clues to
our understanding of CV evolution.
ACKNOWLEDGMENTS
To the memory of Emilios Harlaftis.
AA thanks the Royal Thai Government for a stu-
dentship. BTG was supported by a PPARC Advanced
Fellowship, respectively. MAPT is supported by NASA
LTSA grant NAG-5-10889. RS is supported by the
Deutsches Zentrum für Luft und Raumfahrt (DLR) GmbH
under contract No. FKZ 50 OR 0404. AS is supported
by the Deutsche Forschungsgemeinschaft through grant
Schw536/20-1. The HQS was supported by the Deutsche
Forschungsgemeinschaft through grants Re 353/11 and
Re353/22.
This paper includes data taken at The McDonald Ob-
servatory of The University of Texas at Austin. It is also
based in part on observations obtained at the German-
Spanish Astronomical Center, Calar Alto, operated by
the Max-Planck-Institut für Astronomie, Heidelberg, jointly
with the Spanish National Commission for Astronomy; on
observations made at the 1.2m telescope, located at Kry-
oneri Korinthias, and owned by the National Observatory of
Athens, Greece; on observations made with the Isaac New-
ton Telescope, which is operated on the island of La Palma
by the Isaac Newton Group in the Spanish Observatorio del
Roque de los Muchachos of the Instituto de Astrof́ısica de
Canarias (IAC); on observations made with the 1.2m tele-
scope at the Fred Lawrence Whipple Observatory, a facility
of the Smithsonian Institution; on observations made with
the IAC80 telescope, operated on the island of Tenerife by
the IAC in the Spanish Observatorio del Teide; on observa-
tions made with the OGS telescope, operated on the island
of Tenerife by the European Space Agency, in the Spanish
Observatorio del Teide of the IAC; on observations made
with the Nordic Optical Telescope, operated on the island
of La Palma jointly by Denmark, Finland, Iceland, Norway,
and Sweden, in the Spanish Observatorio del Roque de los
Muchachos of the IAC; and on observations made with the
NASA/ESA Hubble Space Telescope, obtained at the Space
Telescope Science Institute, which is operated by the Associ-
ation of Universities for Research in Astronomy, Inc., under
NASA contract NAS 5-26555.
This publication makes use of data products from the
Two Micron All Sky Survey, which is a joint project of the
University of Massachusetts and the Infrared Processing and
Analysis Center/California Institute of Technology, funded
by the National Aeronautics and Space Administration and
the National Science Foundation.
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c© 2007 RAS, MNRAS 000,
	Introduction
	Observations and data reduction
	Identification
	Optical photometry
	Optical spectroscopy
	HST/STIS far-ultraviolet spectroscopy
	Photometric periods
	HS0129+2933, HS0220+0603, and HS0455+8315
	HS0805+3822
	HS1813+6122
	Spectroscopic analysis
	An overabundance of nitrogen in HS0220+0603
	Radial velocities
	The orbital period of HS1813+6122
	Trailed spectra 
	The FUV spectrum of HS0455+8315
	SWSex membership
	The SW Sex stars in the context of CV evolution
	How are the SW Sex stars discovered as CVs?
	The role of the SW Sex stars in the big family of nova-like CVs
	CV evolution and the SW Sex stars
	A rich phenomenology to explore
