The ISLAnds Project III: Variable Stars in Six Andromeda Dwarf
  Spheroidal Galaxies
  We present a census of variable stars in six M31 dwarf spheroidal satellites
observed with the Hubble Space Telescope. We detect 870 RR Lyrae (RRL) stars in
the fields of And I (296), II (251), III (111), XV (117), XVI (8), XXVIII (87).
We also detect a total of 15 Anomalous Cepheids, three Eclipsing Binaries, and
seven field RRL stars compatible with being members of the M31 halo or the
Giant Stellar Stream. We derive robust and homogeneous distances to the six
galaxies using different methods based on the properties of the RRL stars.
Working with the up-to-date set of Period-Wesenheit ($I$, $B$ - $I$) relations
published by Marconi et al., we obtain distance moduli of $\mu_0$ = [24.49,
24.16, 24.36, 24.42, 23.70, 24.43] mag (respectively), with systematic
uncertainties of 0.08 mag and statistical uncertainties $<$ 0.11 mag. We have
considered an enlarged sample of sixteen M31 satellites with published
variability studies, and compared their pulsational observables (e.g., periods,
amplitudes), with those of fifteen Milky Way satellites for which similar data
are available. The properties of the (strictly old) RRL in both satellite
systems do not show any significant difference. In particular, we found a
strikingly similar correlation between the mean period distribution of the
fundamental RRL pulsators (RRab) and the mean metallicities of the galaxies.
This indicates that the old RRL progenitors were similar at the early stage in
the two environments, suggesting very similar characteristics for the earliest
stages of evolution of both satellite systems.
