Variable stars and stellar populations in Andromeda XXI: II. Another
  merged galaxy satellite of M31?
  B and V time-series photometry of the M31 dwarf spheroidal satellite
Andromeda XXI (And XXI) was obtained with the Large Binocular Cameras at the
Large Binocular Telescope. We have identified 50 variables in And XXI, of which
41 are RR Lyrae stars (37 fundamental-mode RRab, and 4 first-overtone RRc,
pulsators) and 9 are Anomalous Cepheids (ACs). The average period of the RRab
stars (<Pab> = 0.64 days) and the period-amplitude diagram place And~XXI in the
class of Oosterhoff II - Oosterhoff-Intermediate objects. From the average
luminosity of the RR Lyrae stars we derived the galaxy distance modulus of
(m-M)$_0$=$24.40\pm0.17$ mag, which is smaller than previous literature
estimates, although still consistent with them within 1 $\sigma$. The galaxy
color-magnitude diagram shows evidence for the presence of three different
stellar generations in And~XXI: 1) an old ($\sim$ 12 Gyr) and metal poor
([Fe/H]=$-$1.7 dex) component traced by the RR Lyrae stars; 2) a slightly
younger (10-6 Gyr) and more metal rich ([Fe/H]=$-$1.5 dex) component populating
the red horizontal branch, and 3) a young age ($\sim$ 1 Gyr) component with
same metallicity, that produced the ACs. Finally, we provide hints that And~XXI
could be the result of a minor merging event between two dwarf galaxies.
