BLUE STRAGGLERS AS REMNANTS OF STELLAR MERGERS - THE DISCOVERY OF SHORT-PERIOD ECLIPSING BINARIES IN THE GLOBULAR-CLUSTER NGC-5466

We have obtained numerous V and B CCD images of the central region of the metal-poor globular cluster NGC 5466 to search for short-period variables among the known blue stragglers in the cluster. Of the nine variable blue stragglers discovered in this survey, three are eclipsing binaries with periods between 0.298 and 0.511 days. The remaining six variable blue stragglers are pulsating SX Phe stars, and their properties will be described in detail elsewhere. Of the three eclipsing systems, two are contact binaries (W UMa stars), while the third is a very short-period semidetached (Algol) or fully detached binary. The photometric properties of all three stars are consistent with cluster membership. We compare the number of close binaries and the total number of blue stragglers in NGC 5466 with theoretical estimates of the timescales required for contact systems to merge into single stars and the lifetimes of blue stragglers. This comparison is consistent with the claim that all of the noneclipsing blue stragglers in NGC 5466 formed as the result of mergers of the components in close binary systems, although the uncertainties in our analysis are sufficiently large that we cannot rule out the possibility that other mechanisms have produced some fraction of the blue stragglers in the cluster. The observed number of close binaries among well-studied blue stragglers in intermediate-age and old open clusters and globular clusters is not inconsistent with the merger hypothesis.