The formation of black hole low-mass X-ray binaries: Through case B or case C mass transfer?

The formation of low-mass X-ray binaries containing a rather massive (M gre ater than or similar to7 M-circle dot) black hole is problematic because in most recent stellar evolutionary calculations the immediate progenitors of these black holes (Wolf-Rayet stars) lose so much mass via their stellar w ind that their final masses are well below the observed black hole masses. We discuss the recently proposed solution that these binaries are formed th rough case C mass transfer (i.e. mass transfer after core helium burning is completed), avoiding a long Wolf-Rayet phase and thus significant mass los s. We show that only some of the currently available models for the evoluti on of massive stars allow this formation channel. We also investigate the e ffect of the downward revised Wolf-Rayet mass-loss rate as is suggested by observations, and conclude that in that case Wolf-Rayet stars end their liv es with significantly higher masses than previously found and may be able t o form a black holes.