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^11,12,13,14.

白矮星是致密、冷却的恒星余烬，主要由碳和氧组成^{1 ，或在较高初始恒星质量2}时由氧和氖（含有百分之几的碳）组成。这些恒星核心被氦壳包裹着，而氦壳通常又被一层氢包围，通常禁止直接观察内部成分。然而，在相当大一部分白矮星^{3,4的表面观察到碳，有时还带有微量的氧，并且被认为是由深氦对流区5,6}从核心挖出的。在这些物体中，仅发现微量的氢^7,8，因为预计大量氢会抑制外壳内的氢-氦对流混合⁹。_{我们报告了 WD J055134.612+ 413531.09}的鉴定，这是一颗超大质量（1.14 个太阳质量（M⊙））白矮星，具有独特的碳氢混合大气（原子比 C∕H = 0.15）。我们对外壳和内部的分析表明，氢和氦的总质量分数必须比单星演化¹⁰的预测低几个数量级：分别小于 10 ^-9.5和 10 ^-7.0。由于快速运动学（129 ± 5 km s ^-1相对于当地的静止标准），大质量和特殊的包络成分，我们认为 WD J0551+4135 与紧密双星系统中两颗白矮星合并形成的^11,12,13,14一致。