An ultra-massive white dwarf with a mixed hydrogen-carbon atmosphere as a likely merger remnant

White dwarfs are dense, cooling stellar embers consisting mostly of carbon and oxygen(1), or oxygen and neon (with a few per cent carbon) at higher initial stellar masses(2). 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(3,4), sometimes with traces of oxygen, and is thought to be dredged up from the core by a deep helium convection zone(5,6). In these objects, only traces of hydrogen are found(7,8), as large masses of hydrogen are predicted to inhibit hydrogen-helium convective mixing within the envelope(9). We report the identification of WD J055134.612+413531.09, an ultra-massive (1.14 solar masses (M-circle dot)) white dwarf with a unique carbon-hydrogen mixed atmosphere (atomic ratio C/H = 0.15). 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(10): less than 10(-9.5) and 10(-7.0), respectively. Due to the fast kinematics (129 +/- 5 km s(-1) relative to the local standard of rest), large mass and peculiar envelope composition, we argue that WD J0551+4135 is consistent with formation from the merger of two white dwarfs in a tight binary system(11-14). A massive white dwarf (WD) with unusually low hydrogen and helium abundances, undetectable oxygen and high C/H poses a challenge to single-star evolution models. However, this object fits with expectations of a WD-WD merger product.