RR Lyrae stars in the newly discovered ultra-faint dwarf galaxy
  Centaurus I
  We report the detection of three RR Lyrae (RRL) stars (two RRc and one RRab)
in the ultra-faint dwarf (UFD) galaxy Centaurus I (CenI) and two Milky Way (MW)
$\delta$ Scuti/SX Phoenicis stars based on multi-epoch $giz$ DECam
observations. The two RRc stars are located within 2 times the half-light
radius (r$_h$) of Cen I, while the RRab star (CenI-V3) is at $\sim6$ r$_h$. The
presence of three distant RRL stars clustered this tightly in space represents
a 4.7$\sigma$ excess relative to the smooth distribution of RRL in the Galactic
halo. Using the newly detected RRL stars, we obtain a distance modulus to Cen I
of $\mu_0 = 20.354 \pm 0.002$ mag ($\sigma=0.03$ mag), a heliocentric distance
of D$_\odot = 117.7 \pm 0.1$ kpc ($\sigma=1.6$ kpc), with systematic errors of
$0.07$ mag and $4$ kpc. The location of the Cen I RRL stars in the Bailey
diagram is in agreement with other UFD galaxies (mainly Oosterhoff II).
Finally, we study the relative rate of RRc+RRd (RRcd) stars ($f_{cd}$) in UFD
and classical dwarf galaxies. The full sample of MW dwarf galaxies gives a mean
of $f_{cd} = 0.28$. While several UFD galaxies, such as Cen I, present higher
RRcd ratios, if we combine the RRL populations of all UFD galaxies, the RRcd
ratio is similar to the one obtained for the classical dwarfs ($f_{cd}$ $\sim$
0.3). Therefore, there is no evidence for a different fraction of RRcd stars in
UFD and classical dwarf galaxies.
