  Massive binary evolution models are needed to predict massive star
populations in star forming galaxies, the supernova diversity, and the number
and properties of gravitational wave sources. Such models are often computed
using so called rapid binary evolution codes, which approximate the evolution
of the binary components based on detailed single star models. However, about
one third of the interacting massive binary stars undergo mass transfer during
core hydrogen burning (Case A mass transfer), whose outcome is difficult to
derive from single star models. Here, we use a large grid of detailed binary
evolution models for primaries in the initial mass range 10 to 40 Solar masses
of LMC and SMC composition, to derive analytic fits for the key quantities
needed in rapid binary evolution codes, i.e., the duration of core hydrogen
burning, and the resulting donor star mass. Systems with shorter orbital
periods produce up to 50% lighter stripped donors and have a up to 30% larger
lifetime than wider systems. We find that both quantities depend strongly on
the initial binary orbital period, but that the initial mass ratio and the mass
transfer efficiency of the binary have little impact on the outcome. Our
results are easily parameterisable and can be used to capture the effects of
Case A mass transfer more accurately in rapid binary evolution codes.
