  We introduce a set of stellar models for massive stars whose evolution has
been affected by mass transfer in a binary system, at a range of metallicities.
As noted by other authors, the effect of such mass transfer is frequently more
than just rejuvenation. We find that, whilst stars with convective cores which
have accreted only H-rich matter rejuvenate as expected, those stars which have
accreted He-rich matter (for example at the end stages of conservative mass
transfer) evolve in a way that is qualitatively similar to rejuvenated stars of
much higher metallicity. Thus the effects of non-conservative evolution depend
strongly on whether He-rich matter is amongst the portion accreted or ejected.
This may lead to a significant divergence in binary evolution paths with only a
small difference in initial assumptions. We compare our models to observed
systems and find approximate formulae for the effect of mass accretion on the
effective age and metallicity of the resulting star.
