2001NewA....6..331B

We study the formation of low-mass black hole X-ray binaries with main sequence companions that have formed through case C mass transfer (mass transfer following the helium core burning phase of the black hole progenitor). We identify these objects with the observed soft X-ray transients. Although this scenario requires a set of fine tuned conditions, we are able to produce a current Galactic population of ∼2000 objects, in agreement with estimates based on observations. The narrow interval in initial separations leading to case C mass transfer, combined with the allowed narrow range of separations after the common envelope evolution, constrains the common envelope efficiency in this scenario: λαce~0.2-0.5.