SDSS unveils a population of intrinsically faint cataclysmic variables
  at the minimum orbital period
  We discuss the properties of 137 cataclysmic variables (CVs) which are
included in the Sloan Digital Sky Survey (SDSS) spectroscopic data base, and
for which accurate orbital periods have been measured. 92 of these systems are
new discoveries from SDSS and were followed-up in more detail over the past few
years. 45 systems were previously identified as CVs because of the detection of
optical outbursts and/or X-ray emission, and subsequently re-identified from
the SDSS spectroscopy. The period distribution of the SDSS CVs differs
dramatically from that of all the previously known CVs, in particular it
contains a significant accumulation of systems in the orbital period range
80--86 min. We identify this feature as the elusive "period minimum spike"
predicted by CV population models, which resolves a long-standing discrepancy
between compact binary evolution theory and observations. We show that this
spike is almost entirely due to the large number of CVs with very low accretion
activity identified by SDSS. The optical spectra of these systems are dominated
by emission from the white dwarf photosphere, and display little or no
spectroscopic signature from the donor stars, suggesting very low-mass
companion stars. We determine the average absolute magnitude of these
low-luminosity CVs at the period minimum to be <M_g>=11.6+-0.7. Comparison of
the SDSS CV sample to the CVs found in the Hamburg Quasar Survey and the
Palomar Green Survey suggests that the depth of SDSS is the key ingredient
resulting in the discovery of a large number of intrinsically faint
short-period systems.
