Transients among Binaries with Evolved Low–mass Companions

We show that stable disk accretion should be very rare among low–mass X–ray binaries and cataclysmic variables whose evolution is driven by the nuclear expansion of the secondary star on the first giant branch. Stable accretion is confined to neutron– star systems where the secondary is still relatively massive, and some supersoft white dwarf accretors. All other systems, including all black–hole systems, appear as soft X– ray transients or dwarf novae. All long–period neutron–star systems become transient well before most of the envelope mass is transferred, and remain transient until envelope exhaustion. This complicates attempts to compare the numbers of millisecond pulsars in the Galactic disk with their LMXB progenitors, and also means that the pulsar spin rates are fixed in systems which are transient rather than steady, contrary to common assumption. The long–period persistent sources Sco X–2, LMC X–2, Cyg X–2 and V395 Car must have minimum companion masses M2 ∼> 0.75M⊙ if they contain neutron stars, and still larger M2 if they contain black holes. The neutron–star transient GRO J17442844 must have M2 ∼< 0.87M⊙. The existence of any steady sources at all at long periods supports the ideas that (a) the accretion disks in many, if not all, LMXBs are strongly irradiated by the central source, and (b) mass transfer is thermally unstable in long–period supersoft X–ray sources. Subject headings: accretion, accretion disks — binaries: close — black hole physics – instabilities