2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/141443A number of X-ray binaries (neutron stars or black holes accreting from a companion star) have such short orbital periods that ordinary, hydrogen rich, stars do not fit in. Instead the mass-losing star must be a compact, evolved star, leading to the transfer of hydrogen deficient material to the neutron star. I discuss the current knowledge of these objects, with focus on optical spectroscopy.in proceedings "Hydrogen-Deficient Stars" (eds. K Werner & T Rauch), ASP Conf SeriesAstrophysicstext