We present B,V,I time-series photometry of the Coma Berenices dwarf spheroidal galaxy , a faint Milky Way satellite , recently discovered by the Sloan Digital Sky Survey .
We have obtained V,B - V and V,V - I color-magnitude diagrams that reach V \sim 23.0 - 23.2 mag showing the galaxy turnoff at V \sim 21.7 mag , and have performed the first study of the variable star population of this new Milky Way companion .
Two RR Lyrae stars ( a fundamental-mode -RRab- and a first overtone -RRc- pulsator ) and a short period variable with period P =0.12468 days were identified in the galaxy .
The RRab star has a rather long period of P _ { ab } = 0.66971 days and is about 0.2 mag brighter than the RRc variable and other non-variable stars on the galaxy horizontal branch .
In the period-amplitude diagram the RRab variable falls closer to the loci of Oosterhoff type-II systems and evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3 .
The average apparent magnitude of the galaxy horizontal branch , \left < V _ { HB } \right > = 18.64 \pm 0.04 mag , leads to a distance modulus for the Coma dSph \mu _ { 0 } = 18.13 \pm 0.08 mag , corresponding to a distance d =42 ^ { +2 } _ { -1 } ~ { } kpc , by adopting a reddening E ( B - V ) = 0.045 \pm 0.015 mag and a metallicity [ Fe/H ] = -2.53 \pm 0.05 dex .