  Interactions between massive stars in binaries are thought to be responsible
for much of the observed diversity of supernovae. As surveys probe rarer
populations of events, we should expect to see supernovae arising from
increasingly uncommon progenitor channels. Here we examine a scenario in which
massive stars merge after they have both formed a hydrogen-exhausted core. We
suggest this could produce stars which explode as pair-instability supernovae
(PISNe) with significantly more hydrogen, at a given metallicity, than in
single-star models with the same pre-explosion oxygen-rich core mass. We
investigate the subset of those stellar mergers which later produce pulsational
PISNe, and estimate that the rate of such post-merger, hydrogen-rich
pulsational PISNe could approach a few in a thousand of all core-collapse
supernovae. The nature and predicted rate of such hydrogen-rich pulsational
PISNe are reminiscent of the very unusual supernova iPTF14hls. For plausible
assumptions, PISNe from similar mergers might dominate the rate of PISNe in the
local Universe.
