  The light curve of GW170817 is surprisingly blue and bright. Assuming that
the event is a binary neutron star merger, we argue that blueness and
brightness of the light curve is the result of ejecta that contains an
substantial amount of thermal energy. To achieve this, the ejecta must be
reheated at a substantial distance (1 to 2000 solar radii) from the merger to
avoid losing the energy to adiabatic cooling. We show that this reheating can
occur if the merger occurs in a hierarchical triple system where the outer star
has evolved and filled its Roche lobe. The outer star feeds mass to the inner
binary, forming a circumbinary disc, driving the inner binary to merge. Because
the outer star fills its Roche lobe, a substantial fraction of the dynamical
ejecta collides with the evolved star, reheating the ejecta in the process. We
suggest that the process of mass transfer in heirarchical triples tends to form
coplanar triple systems such as PSR J0337+1715, and may provide electromagnetic
counterparts to binary black hole mergers.
