The existence of ultra-massive white dwarfs (M>1.2 Msun) is of particular interest for several fields, ranging from the physical 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. Here we report on the discovery and new observations of a white dwarf with mass 1.28+/-0.05 Msun in a peculiar binary system resulting from common envelope evolution.