VARIABLE-STARS WITHIN 25 ARCMIN OF THE LMC GLOBULAR-CLUSTER NGC 2210

Based on a photometric analysis of 63 photographic plates taken between 1975 and 1985 with the CTIO 1.5 and 4 m telescopes, and new photoelectric and CCD photometry, pulsational characteristics are presented for 52 newly discovered field RR Lyrae variables within 0.5-degrees of the LMC globular cluster NGC 22 1 0. The surface density of the field RR Lyrae stars is 80 per square degree, which is as high as that found in Graham's NGC 1783 field, and higher than that found in several outer halo fields of the LMC. Eleven RR Lyrae stars located within the tidal radius of the cluster, and 33 non-RR Lyrae field variables are also discussed. The mean periods of the 40 ab-type and 12 c-type field RR Lyrae stars, [P(ab)]=0.576 +/- 0.057 day and [P(c)]=0.345 +/- 0.049 day, fall between the values for the two Oosterhoff groups in our Galaxy. Since such mean periods are also found in the NGC 1783 and NGC 2257 regions of the LMC, we conclude that such an intermediate Oosterhoff type is a general characteristic of LMC field RR Lyrae stars. Based on a comparison of the observed period-amplitude (P-A) relation with those for the RR Lyrae stars in the galactic globular clusters M3 and M15, the mean metallicity of the NGC 2210 field RR Lyrae stars appears to be [Fe/H]= -1.8. Support for this conclusion comes also from the observed ratio of RRab to RRc stars. The mean [B] for the field variables, 19.56 +/- 0.02 mag, when taken with an assumed distance to the LMC of 50 +/- 3 kpc, and a line-of-sight reddening E (B - V)=0.08 +/- 0.01, suggests that the absolute magnitude of the field RR Lyrae stars is M(B)=0.75 +/- 0.20 mag.