  Half or more of stars more massive than our Sun are orbited by a companion
star in a binary system. Many binaries have short enough orbits that the
evolution of both stars is greatly altered by an exchange of mass and angular
momentum between the stars. Such mass transfer is highly likely on the
asymptotic giant branch (AGB) because this is when a star is both very large
and has strong wind mass loss. Direct mass transfer truncates the AGB, and its
associated nucleosynthesis, prematurely compared to in a single star. In wide
binaries we can probe nucleosynthesis in the long-dead AGB primary star by
today observing its initially lower-mass companion. The star we see now may be
polluted by ejecta from the primary either through a wind or Roche-lobe
overflow. We highlight recent quantitative work on nucleosynthesis in (ex-)AGB
mass-transfer systems, such as carbon and barium stars, the link between binary
stars and planetary nebulae, and suggest AGB stars as a possible source of the
enigmatic element, lithium.
