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 ⊙ )) 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 .