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, that we have imaged with the
Large Binocular Cameras of the Large Binocular Telescope. The field of
Andromeda XXV is found to contain 63 variable stars, for which we present light
curves and characteristics of the light variation (period, amplitudes,
variability type, mean magnitudes, etc.). The sample includes 58 RR Lyrae
variables (46 fundamental-mode $-$ RRab, and 12 first-overtone $-$RRc,
pulsators), three anomalous Cepheids, one eclipsing binary system and one
unclassified variable. The average period of the RRab stars ($\langle Pab
\rangle$ = 0.60 $\sigma=0.04$ 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)$_0$=$24.63\pm0.17$ 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 $\sim$ 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 number and characteristics of the pulsating stars in the
M31 satellites analyzed so far for variability.
