The incidence of stellar mergers and mass gainers among massive stars
  Because the majority of massive stars are born as members of close binary
systems, populations of massive main-sequence stars contain stellar mergers and
products of binary mass transfer. We simulate populations of massive stars
accounting for all major binary evolution effects based on the most recent
binary parameter statistics and extensively evaluate the effect of model
uncertainties.
  Assuming constant star formation, we find that $8^{+9}_{-4}\,\%$ of a sample
of early type stars to be the product of a merger resulting from a close binary
system. In total we find that $30^{+10}_{-15}\,\%$ of massive main-sequence
stars are the product of binary interaction.
  We show that the commonly adapted approach to minimize the effects of
binaries on an observed sample by excluding systems detected as binaries
through radial velocity campaigns can be counterproductive. Systems with
significant radial velocity variations are mostly pre-interaction systems.
Excluding them substantially enhances the relative incidence of mergers and
binary products in the non radial velocity variable sample.
  This poses a challenge for testing single stellar evolutionary models. It
also raises the question of whether certain peculiar classes of stars, such as
magnetic O-stars, are the result of binary interaction and it emphasizes the
need to further study the effect of binarity on the diagnostics that are used
to derive the fundamental properties (star-formation history, initial mass
function, mass to light ratio) of stellar populations nearby and at high
redshift.
