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 oxygen¹, or oxygen and neon (with a few per cent carbon) at higher initial stellar masses². 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 envelope⁹. We report the identification of WD J055134.612+413531.09, an ultra-massive (1.14 solar masses (M_⊙)) 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 evolution¹⁰: 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.