We present spectroscopy of the eclipsing recurrent nova U Sco .
The radial velocity semi-amplitude of the primary star was found to be K _ { W } = 93 \pm 10 km s ^ { -1 } from the motion of the wings of the HeII \ > \lambda 4686 Å \ > emission line .
By detecting weak absorption features from the secondary star , we find its radial velocity semi-amplitude to be K _ { R } = 170 \pm 10 km s ^ { -1 } .
From these parameters , we obtain a mass of M _ { 1 } = 1.55 \pm 0.24 M _ { \odot } for the white dwarf primary star and a mass of M _ { 2 } = 0.88 \pm 0.17 M _ { \odot } for the secondary star .
The radius of the secondary is calculated to be R _ { 2 } = 2.1 \pm 0.2 R _ { \odot } , confirming that it is evolved .
The inclination of the system is calculated to be i = 82.7 ^ { \circ } \pm 2.9 ^ { \circ } , consistent with the deep eclipse seen in the lightcurves .
The helium emission lines are double-peaked , with the blue-shifted regions of the disc being eclipsed prior to the red-shifted regions , clearly indicating the presence of an accretion disc .
The high mass of the white dwarf is consistent with the thermonuclear runaway model of recurrent nova outbursts , and confirms that U Sco is the best Type Ia supernova progenitor currently known .
We predict that U Sco is likely to explode within \sim 700 000 years .