A SEARCH FOR DOUBLE-MODE RR LYRAE STARS IN THE GLOBULAR-CLUSTERS M80, M9, AND NGC-2298

Observations of RR Lyrae variables near the transition between fundamental and first-overtone mode in the Oosterhoff type II (Oo II) globular clusters M80, M9, and NGC 2298 have been analyzed in a search for double-mode pulsators (RRd stars). These three clusters were selected because they have metal abundances that are intermediate between those of the Oo II clusters and the Oo I clusters that are known to have RRd stars. Any RRd stars identified in these three clusters would be useful for verification of a mass-metallicity relation. The analysis was based on published observations for all three clusters, and in addition, 44 CCD frames of M9 and 32 photographs of NGC 2298, all obtained with the University of Toronto telescope at Las Campanas. No new RRd stars were discovered. It is noted that the only globular clusters known to have RRd stars are the most metal poor Oo I clusters and metal poor Oo II clusters. Published observations of the RRd stars in M15 have been analyzed in order to determine accurate periods and period ratios for stars whose periods were previously considered to be uncertain.