A rich harvest of RR Lyrae stars has been identified for the first time in B514 , a metal-poor ( [ Fe/H ] \sim –1.95 \pm 0.10 dex ) globular cluster of the Andromeda galaxy ( M31 ) , based on Hubble Space Telescope Wide Field Planetary Camera 2 and Advanced Camera for Surveys time-series observations .
We have detected and derived periods for 89 RR Lyrae stars ( 82 fundamental-mode - RRab - and 7 first-overtone - RRc - pulsators , respectively ) among 161 candidate variables identified in the cluster .
The average period of the RR Lyrae variables ( \langle Pab \rangle = 0.58 days and \langle Pc \rangle = 0.35 days , for RRab and RRc pulsators , respectively ) and the position in the period-amplitude diagram both suggest that B514 is likely an Oosterhoff type I cluster .
This appears to be in disagreement with the general behaviour of the metal-poor globular clusters in the Milky Way , which show instead Oosterhoff type II pulsation properties .

The average apparent magnitude of the RR Lyrae stars sets the mean level of the cluster horizontal branch at \langle V ( RR ) \rangle = 25.18 \pm 0.02 ( \sigma =0.16 mag , on 81 stars ) .
By adopting a reddening E ( B - V ) = 0.07 \pm 0.02 mag ( Schlegel et al . 23 ) , the above metallicity and M _ { V } =0.44 \pm 0.05 mag for the RR Lyrae variables of this metallicity ( Clementini et al . 9 ) , we derive a distance modulus of \mu _ { 0 } = 24.52 \pm 0.08 mag , corresponding to a distance of about 800 \pm 30 kpc , based on a value of M _ { V } that sets \mu _ { 0 } ( LMC ) =18.52 .