Black hole binaries and X-ray transients

We consider transient behavior in low-mass X-ray binaries (LMXBs). We show that if this results from a disk instability, the secondary star must be significantly evolved when mass transfer starts, particularly if the primary is a neutron star. For P less than or similar to 2 days, most neutron star systems will be persistent X-ray sources, whereas the slower orbital evolution of black hole systems means that most of them are likely to be transient. Both types of transient system must have extreme mass ratios (<0.1). For orbital periods P greater than or similar to 2 days, most LMXBs will be transient regardless of whether the primary is a neutron star or a Mack hole.