An Ultramassive, Fast-Spinning White Dwarf in a Peculiar Binary System

White dwarfs typically have masses in a narrow range centered at about 0.6 solar mass (M-circle dot). Only a few ultramassive white dwarfs (mass > 1.2 M-circle dot) are known. Those in binary systems are of particular interest, because a small amount of accreted mass could drive them above the Chandrasekhar limit, beyond which they become gravitationally unstable. Using data from the x-ray multimirror mission (XMM)-Newton satellite, we show that the x-ray pulsator RX J0648.0-4418 is a white dwarf with mass > 1.2 M-circle dot, based on dynamical measurements only. This ultramassive white dwarf in a post-common envelope binary with a hot subdwarf can reach the Chandrasekhar limit, and possibly explode as a type Ia supernova, when its helium-rich companion will transfer mass at an increased rate through Roche lobe overflow.