  We consider evolutionary models for the population of short-period (<10 hr)
low-mass black-hole binaries (LMBHB) and compare them with observations of soft
X-ray transients (SXT). Evolution of LMBHB is determined by nuclear evolution
of the donors and/or orbital angular momentum loss due to magnetic braking by
the stellar wind of the donors and gravitational wave radiation. We show that
the absence of observed stable luminous LMBHB implies that upon RLOF by the
low-mass donor angular momentum losses are substantially reduced with respect
to the Verbunt and Zwaan "standard" prescription for magnetic braking. Under
this assumption masses and effective temperatures of the model secondaries of
LMBHB are in a satisfactory agreement with the masses and effective
temperatures (as inferred from their spectra) of the observed donors in LMBHB.
Theoretical mass-transfer rates in SXTs are consistent with the observed ones
if one assumes that accretion discs in these systems are truncated ("leaky").
We find that the population of short-period SXT is formed mainly by systems
which had unevolved or slightly evolved (X_c > 0.35) donors at the RLOF. Longer
period (0.5 - 1 day) SXT might descend from systems with initial donor mass
about 1 solar and X_c < 0.35. It is unnecessary to invoke donors with almost
hydrogen-depleted cores to explain the origin of LMBHB. Our models suggest that
a very high efficiency of common-envelopes ejection is necessary to form LMBHB,
unless currently commonly accepted empirical estimates of mass-loss rates by
winds for pre-WR and WR-stars are significantly over-evaluated.

Astrophysics