Abstract
We present results on the study of the variable stars and stellar populations of the Andromeda IX (And IX) dwarf spheroidal companion of M31, based on g', r' time-series photometry we obtained as part of our ESO Large Program 186.D-2013 at the Gran Telescopio Canarias (GTC; PI G. Clementini). More than twenty RR Lyrae stars were discovered in And IX and well sampled light curves were obtained, with an accuracy of the individual data points to better than 0.1 mag even at the RR Lyrae minimum light, which is around g'~25.5 mag. The color magnitude diagram (CMD) of And IX obtained by reduction of the single epoch data reaches g'~26.3-26.4 mag, while by stacking the 26 pairs of g', r' exposures available for the galaxy we were able to reach g'~27.3 mag. This is 2 mag fainter than presently found in the literature. And IX shows a prominent red giant branch (RGB). The horizontal branch (HB) is clearly visible at g'~25.3 mag, although it is rather contaminated by field stars. The galaxy CMD is well fitted by the mean ridge lines of a metal poor globular cluster such as NGC 2419 ([Fe/H]=-2.2 on the Carretta et al. 2009 metallicity scale).
