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We present a B, V color-magnitude diagram (CMD) of the Milky Way dwarfsatellite Ursa Major II (UMa II), spanning the magnitude range from V ~ 15 to V~ 23.5 mag and extending over a 18 {\times} 18 arcmin2 area centered on thegalaxy. Our photometry goes down to about 2 magnitudes below the galaxy's mainsequence turn-off, that we detected at V ~ 21.5 mag. We have discovered abona-fide RR Lyrae variable star in UMa II, which we use to estimate aconservative dereddened distance modulus for the galaxy of (m-M)0 =17.70{\pm}0.04{\pm}0.12 mag, where the first error accounts for theuncertainties of the calibrated photometry, and the second reflects our lack ofinformation on the metallicity of the star. The corresponding distance to UMaII is 34.7 {\pm} 0.6 ({\pm} 2.0) kpc. Our photometry shows evidence of a spreadin the galaxy subgiant branch, compatible with a spread in metal abundance inthe range between Z=0.0001 and Z=0.001. Based on our estimate of the distance,a comparison of the fiducial lines of the Galactic globular clusters (GCs) M68and M5 ([Fe/H]=-2.27 {\pm} 0.04 dex and -1.33 {\pm} 0.02 dex, respectively),with the position on the CMD of spectroscopically confirmed galaxy members, maysuggest the existence of stellar populations of different metal abundance/agein the central region of UMa II.

Stellar Archaeology in the Galactic halo with the Ultra-Faint Dwarfs: VI. Ursa Major II