We derive the mass of the white dwarf in the eclipsing recurrent nova U Sco from the radial velocity semi–amplitudes of the primary and secondary stars .
Our results give a high white dwarf mass of M _ { 1 } = 1.55 \pm 0.24 M _ { \odot } , consistent with the thermonuclear runaway model of recurrent nova outbursts .
We confirm that U Sco is the best Type Ia supernova progenitor known , and predict that the time to explosion is within \sim 700 000 years .