Mass from a third star: transformations of close compact-object binaries
  within hierarchical triples
  Close-orbit binaries consisting of two compact objects are a center of
attention because of the detection of gravitational-radiation-induced mergers.
The creation of close, compact-object binaries involves physical processes that
are not yet well understood; there are open questions about the manner in which
two compact objects come to be close enough to merge within a Hubble time. Here
we explore an important, and likely common physical process: mass transfer from
a third star in a wider, hierarchical orbit. Mass added to the close binary's
components can reduce the time to merger and can even change the nature of an
accretor, transforming a white dwarf to a neutron star and/or a neutron star to
a black hole. Some accreting white dwarfs in close binaries may even explode as
Type Ia supernovae. Given the ubiquity of higher-order multiples, the
evolutionary channels we lay out may be important pathways to gravitational
mergers, including Type Ia supernovae. Fortunately, these pathways also lead to
testable predictions.
