Variable Stars in the Large Magellanic Cloud: Discovery of Extragalactic W UMa Binaries

Abstract

We observed a field in the disk of the LMC on two consecutive nights in search of rapid variable stars. We have found two pulsating stars of type RRab and delta Sct, and four binary stars, among the latter one sdB or CV below the LMC blue Main Sequence and three very close binary systems on the MS. At least one of the MS binaries, and possibly all three, are the first solar-type (W UMa-type) contact binaries to be detected in any extragalactic system and observed to obey the same Mv = Mv(log P, B-V) calibration as the Galactic systems. Given the selection effects due to small amplitudes at faint magnitudes, the frequency of such binaries in the disk of the LMC with its large spread in population ages is not inconsistent with that in the disk of our Galaxy, and contrasts with the lack of binaries found in earlier observations of the much younger LMC cluster LW55.