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 kms^{-1} from the motion of the wings of the HeIIlambda4686AA 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 kms^{-1}. From these parameters, we obtain a mass of M_1 = 1.55 pm 0.24M_odot for the white dwarf primary star and a mass of M_2 = 0.88 pm 0.17M_odot for the secondary star. The radius of the secondary is calculated to be R_2 = 2.1pm0.2R_odot, confirming that it is evolved. The inclination of the system is calculated to be i = 82.7^circpm2.9^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.
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