  Orbital decay mechanisms argue that double white dwarf mergers are
inevitable, but extremely rare. Whilst some mergers result in explosions, the
survivors re-ignite helium and burn brightly for tens of thousands or millions
of years. Candidate survivors include extreme helium stars, R CrB variables and
various classes of helium-rich subluminous star. Nuclear waste on the
survivors' surfaces provides evidence of the stars' nuclear history prior to
and their nucleosynthesis during the merger. Extensive and deep spectroscopic
surveys offer rich prospects for future discoveries.
