  Stable mass transfer from a massive post-main sequence (post-MS) donor is
thought to be a short-lived event of thermal-timescale mass transfer which
strips the donor star of nearly its entire H-rich envelope, producing a hot
helium star. This long-standing picture is based on stellar models with
Hertzprung gap (HG) donors. Motivated by a finding that in low-metallicity
binaries, post-MS mass transfer may instead be initiated by core-helium burning
(CHeB) donors, we use the MESA code to compute grids of detailed massive binary
models at three metallicities: Solar and that of the Large and the Small
Magellanic Cloud (LMC, SMC). We find that metallicity strongly influences the
course and outcome of mass transfer. We identify two novel types of post-MS
mass transfer: (a) mass exchange on the long nuclear timescale that continues
until the end of the CHeB phase, and (b) rapid mass transfer leading to
detached binaries with mass-losers that are only partially stripped of their
envelopes. In neither (a) or (b) does the donor become a fully stripped helium
star by the end of CHeB. Boundaries between the different types of post-MS mass
transfer are associated with the degree of rapid post-MS expansion and, for a
given metallicity, are sensitive to the assumptions about internal mixing. At
low metallicity, we predict fewer hot fully stripped stars formed through
binary interactions and higher compactness of pre-supernova cores.
Nuclear-timescale post-MS mass transfer suggests a strong preference for
metal-poor host galaxies of ultra-luminous X-ray sources with black-hole (BH)
accretors and massive donors, some of which might be the immediate progenitors
of binary BH mergers. It also implies a population of interacting binaries with
blue and yellow supergiant donors. Partially-stripped stars could potentially
explain the puzzling nitrogen-enriched slowly-rotating (super)giants in the
LMC.
