  Subluminous Type Ia supernovae, such as the Type Iax class prototype SN
2002cx, are described by a variety of models such as the failed detonation and
partial deflagration of an accreting carbon-oxygen white dwarf star, or the
explosion of an accreting, hybrid carbon-oxygen-neon core. These models predict
that bound remnants survive such events with, according to some simulations, a
high kick velocity. We report the discovery of a high proper motion, low-mass
white dwarf (LP 40-365) that travels at a velocity greater than the Galactic
escape velocity and whose peculiar atmosphere is dominated by intermediate-mass
elements. Strong evidence indicates that this partially burnt remnant was
ejected following a subluminous Type Ia supernova event. This supports the
viability of single-degenerate supernova progenitors.
