The mass of the white dwarf in the recurrent nova U Scorpii

We present spectroscopy of the eclipsing recurrent nova U Sco. The radial v elocity semiamplitude of the primary star was found to be K-W = 93 +/- 10 k m s(-1) from the motion of the wings of the He ii lambda 4686-Angstrom emis sion line. By detecting weak absorption features from the secondary star, w e find its radial velocity sen-ii-amplitude to be K-R = 170 +/- 10 km. s(-1 ). From these parameters, we obtain a mass of M-1 = 1.55 +/- 0.24 M-. for t he white dwarf primary star and a mass Of M-2 = 0.88 +/- 0.17 M-. for the s econdary star. The radius of the secondary is calculated to be R-2 = 2.1 +/ - 0.2 R-., confirming that it is evolved. The inclination of the system is calculated to be i = 82.degrees7 +/- 2.degrees9, consistent with the deep e clipse seen in the lightcurves. The helium emission lines are double-peaked , with the blueshifted regions of the disc being eclipsed prior to the reds hifted regions, clearly indicating the presence of an accretion disc. The h igh mass of the white dwarf is consistent with the thermonuclear runaway mo del of recurrent nova outbursts, and confirms that U Sco is the best Type l a supernova progenitor currently known. We predict that U Sco is likely to explode within similar to 700 000 yr.