  The progenitor evolution of the massive X-ray binary Wray 977 is investigated
using new models of massive close binary evolution. These models yield
constraints on the mass limit for neutron star/black hole formation in single
stars, M_BH. We argue for quasi-conservative evolution in this system, and we
find M_BH > 13...21 M_sun from the existence of a neutron star in Wray 977,
with the uncertainty being due to uncertainties in the treatment of convection.
Our results revise earlier published much larger values of M_BH derived from
the parameters of Wray 977. Then, on the basis of a grid of 37 evolutionary
models for massive close binaries with various initial masses, mass ratios and
periods, we derive primary initial-final mass, initial mass-final helium core
mass, and initial mass-final CO-core mass relations for the various mass
transfer cases of close binary evolution. From these models we derive for
single stars that M_BH =< 25 M_sun, independent of whether most black hole
binaries formed through the Case A/B or the Case C binary channel. Using our
grid of binary models, we obtain a consistent scenario for the formation of
black holes in binary systems.
