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 \langle P _ { ab } \rangle = 0.70 d , and \langle P _ { c } \rangle = 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 .