Optical spectra of ultra-compact X-ray binaries

Abstract

We present optical spectroscopy of several (candidate) ultra-compact X-ray binaries (UCXBs) obtained with the ESO VLT and Gemini-North telescopes. In only one of five observed UCXB candidates did we we find evidence for H in its spectrum (4U 1556-60). We find some spectra consistent with C/O discs and one consistent with a He/N accretion disc. We discuss the implications of our findings for our understanding of the formation of UCXBs and the Galactic population of UCXBs. At the moment all studied systems are consistent with having white dwarf donors, the majority being C/O rich.