The pulsating variable star population in DDO210
  We have probed the pulsating variable star content of the isolated Local
Group dwarf galaxy, DDO210 (Aquarius), using archival Advanced Camera for
Surveys/$Hubble$ $Space$ $Telescope$ imaging in the F475W and F814W passbands.
We find a total of 32 RR Lyrae stars (24 ab-type, 8 c-type) and 75 Cepheid
variables. The mean periods of the ab-type and c-type RR Lyrae stars are
calculated to be $\langle$P$_{\mathrm{ab}}\rangle = 0.609\pm0.011$ and
$\langle$P$_{\mathrm{c}}\rangle = 0.359\pm0.025$ days, respectively. The light
curve properties of the fundamental mode RR Lyrae stars yield a mean
metallicity of $\langle$[Fe/H]$\rangle$ = -1.63$\pm$0.11 dex for this ancient
population, consistent with a recent synthetic colour-magnitude diagram
analysis. We find this galaxy to be Oosterhoff-intermediate and lacking in
high-amplitude, short-period ab-type RR Lyrae, consistent with behavior
recently observed for many dwarf spheroidals and ultra-faint dwarfs in the
Local Group. We find a distance modulus of $\mu = 25.07\pm 0.12$ as determined
by the RR Lyrae stars, slightly larger but agreeing with recent distance
estimates from the red giant branch tip. We also find a sizable population of
Cepheid variables in this galaxy. We provide evidence in favor of most if not
all of these stars being short-period classical Cepheids. Assuming all of these
stars to be classical Cepheids, we find that most of these Cepheids are
$\sim$300 Myr old, with the youngest Cepheids being offset from the older
Cepheids and the centre of the galaxy. We conclude that this may have resulted
from a migration of star formation in DDO210.
