  We consider evolutionary models for the population of short-period (<10 hr)
low-mass black-hole binaries (LMBHBs) and compare them with observations of
soft X-ray transients (SXTs). We show that assuming strongly reduced magnetic
braking (as suggested by us before for low-mass semidetached binaries) the
calculated masses and effective temperatures of secondaries are encouragingly
close to the observed masses and effective temperatures (as inferred from their
spectra) of donor stars in short-period LMBHBs. 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 SXTs is formed mainly by systems which had unevolved or
slightly evolved main-sequence donors ($M_2 \lesssim 1.2M_\odot$) with a
hydrogen abundance in the center $X_c > 0.35$ at the Roche-lobe overflow
(RLOF). Longer period (0.5 - 1) day) SXTs might descend from systems with
initial donor masses of about 1 $M_\odot$ and $X_c < 0.35$. Thus, one can
explain the origin of short period LMBHB without invoking donors with cores
almost totally depleted of hydrogen. Our models suggest that, unless the
currently accepted empirical estimates of mass-loss rates by winds for massive
O-stars and Wolf-Rayet stars are significantly over-evaluated, a very high
efficiency of common-envelope ejection is necessary to form short-period
LMBHBs.
