  White dwarfs are dense, cooling stellar embers consisting mostly of carbon
and oxygen, or oxygen and neon (with a few percent carbon) at higher initial
stellar masses. These stellar cores are enveloped by a shell of helium which in
turn is usually surrounded by a layer of hydrogen, generally prohibiting direct
observation of the interior composition. However, carbon is observed at the
surface of a sizeable fraction of white dwarfs, sometimes with traces of
oxygen, and it is thought to be dredged-up from the core by a deep helium
convection zone. In these objects only traces of hydrogen are found as large
masses of hydrogen are predicted to inhibit hydrogen/helium convective mixing
within the envelope. We report the identification of WDJ055134.612+413531.09,
an ultra-massive (1.14 $M_\odot$) white dwarf with a unique hydrogen/carbon
mixed atmosphere (C/H=0.15 in number ratio). Our analysis of the envelope and
interior indicates that the total hydrogen and helium mass fractions must be
several orders of magnitude lower than predictions of single star evolution:
less than $10^{-9.5}$ and $10^{-7.0}$, respectively. Due to the fast kinematics
($129\pm5$ km/s relative to the local standard of rest), large mass, and
peculiar envelope composition, we argue that WDJ0551+4135 is consistent with
formation from the merger of two white dwarfs in a tight binary system.
