  Since close WR+O binaries are the result of a strong interaction of both
stars in massive close binary systems, they can be used to constrain the highly
uncertain mass and angular momentum budget during the major mass transfer
phase. We explore the progenitor evolution of the three best suited WR+O
binaries HD 90657, HD 186943 and HD 211853, which are characterized by a WR/O
mass ratio of $\sim$0.5 and periods of 6..10 days. We are doing so at three
different levels of approximation: predicting the massive binary evolution
through simple mass loss and angular momentum loss estimates, through full
binary evolution models with parametrized mass transfer efficiency, and through
binary evolution models including rotation of both components and a physical
model which allows to compute mass and angular momentum loss from the binary
system as function of time during the mass transfer process. All three methods
give consistently the same answers. Our results show that, if these systems
formed through stable mass transfer, their initial periods were smaller than
their current ones, which implies that mass transfer has started during the
core hydrogen burning phase of the initially more massive star. Furthermore,
the mass transfer in all three cases must have been highly non-conservative,
with on average only $\sim$10% of the transferred mass being retained by the
mass receiving star. This result gives support to our system mass and angular
momentum loss model, which predicts that, in the considered systems, about 90%
of the overflowing matter is expelled by the rapid rotation of the mass
receiver close to the $\Omega$-limit, which is reached through the accretion of
the remaining 10%.
