We report on the discovery of a large number of RR Lyrae variable stars in the moderately metal-rich Galactic globular cluster M62 ( NGC 6266 ) , which places it among the top three most RR Lyrae-rich globular clusters known .
Likely members of the cluster in our studied field , from our preliminary number counts , include \approx 130 fundamental-mode ( RRab ) pulsators , with \langle P _ { ab } \rangle = 0.548 d , and \approx 75 first-overtone ( RRc ) pulsators , with \langle P _ { c } \rangle = 0.300 d. The average periods and the position of the RRab variables with well-defined light curves in the Bailey diagram both suggest that the cluster is of Oosterhoff type I .
However , the morphology of the cluster ’ s horizontal branch ( HB ) is strikingly similar to that of the Oosterhoff type II globular cluster M15 ( NGC 7078 ) , with a dominant blue HB component and a very extended blue tail .
Since M15 and M62 differ in metallicity by about one dex , we conclude that metallicity , at a fixed HB type , is a key parameter determining the Oosterhoff status of a globular cluster and the position of its variables in the Bailey diagram .