Abstract. The existence of ultra-massive white dwarfs (M >1.2 M_⊙) is of particular interest for several fields of astrophysics, ranging from the properties of matter at high density, to the evolution of single and binary stars. Ultra-massive white dwarfs are also relevant as likely progenitors of Type Ia supernovae and millisecond pulsars. I will present the discovery — and new X-ray and optical observations — of a white dwarf with a mass of 1.28±0.05 M_⊙ in a peculiar binary system resulting from common envelope evolution.

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