VARIABLE STARS and STELLAR POPULATIONS in ANDROMEDA XXV. III. A CENTRAL CLUSTER or the GALAXY NUCLEUS?

We present B and V time series photometry of Andromeda XXV, the third galaxy in our program on the Andromeda's satellites, which we have imaged with the Large Binocular Cameras of the Large Binocular Telescope. The field of Andromeda XXV is found to contain 62 variable stars, for which we present light curves and characteristics of the light variation (period, amplitudes, variability type, mean magnitudes, etc.). The sample includes 57 RR Lyrae variables (46 fundamental-mode - RRab, and 11 first-overtone - RRc, pulsators), 3 anomalous Cepheids, 1 eclipsing binary system, and 1 unclassified variable. The average period of the RRab stars (σ = 0.04 days) and the period-amplitude diagram place Andromeda XXV in the class of the Oosterhoff-Intermediate objects. From the average luminosity of the RR Lyrae stars we derive for the galaxy a distance modulus of (m-M)0 = 24.63 ± 0.17 mag. The color-magnitude diagram reveals the presence in Andromeda XXV of a single, metal-poor ([Fe/H] = -1.8 dex) stellar population as old as ∼10-12 Gyr, traced by a conspicuous red giant branch and the large population of RR Lyrae stars. We discovered a spherically shaped high density of stars near the galaxy center. This structure appears to be at a distance consistent with Andromeda XXV and we suggest it could either be a star cluster or the nucleus of Andromeda XXV. We provide a summary and compare the number and characteristics of the pulsating stars in the M31 satellites analyzed so far for variability.