Does the Oosterhoff Dichotomy Exist in The Andromeda Galaxy? I. The case
  of G11
  We present the first evidence that Oosterhoff type II globular clusters exist
in the Andromeda galaxy (M31). On the basis of time-series photometry of the
moderately metal-poor ([Fe/H]\sim -1.6 dex) M31 globular cluster G11, obtained
with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we
detected and derived periods for 14 RR Lyrae stars, of which five are found to
lie inside the cluster tidal radius. They include three fundamental-mode (RRab)
and two first-overtone (RRc) pulsators, with average periods <Pab> = 0.70 d,
and <Pc> = 0.40 d, respectively. These mean periods and the position of the
cluster variable stars in the period-amplitude and period-metallicity diagrams,
all suggest that G11 is likely to be an Oosterhoff type II globular cluster.
This appears to be in agreement with the general behavior of Milky Way globular
clusters with similar metallicity and horizontal branch morphology.
