2010A&A...524A..78R

We report on the first spectroscopic search for quiescent degenerate binaries in globular clusters. Our survey is based on a sample of short-period optical variables that are likely optical counterparts of the quiescent X-ray sources in ω Centauri (NGC 5139) and NGC 6397. The studied candidates have nearly sinusoidal light curves with amplitudes of 0.05-0.12mag in V (0.35mag in one case) and periods of 0.1-1.3 days. This type of variability, most probably from the ellipsoidal effect, has been observed in X-ray novae when they settled into quiescence after an outburst. We find that two of the surveyed systems harbor dim components with masses in excess of 1M_☉, making them attractive targets for future investigations. We also suggest that there are two subpopulations of blue stragglers in ω Centauri, differing in either mass-transfer history or helium content, or even in both at once.