Emission from the Secondary Star in the Old Cv Wz Sge

We present the first detection of the mass donor star in the cataclysmic variable WZ Sge. Phase resolved spectroscopy reveals narrow Balmer emission components from the irradiated secondary star during the 2001 outburst. Its radial velocity curve indicates a systemic velocity of −72± 3 km/s and an apparent velocity amplitude of K2app = 493±10 km/s. Doppler tomography reveals a highly asymmetric accretion disc including a significant bright spot contribution 20 days into the outburst. We estimate the primary radial velocity K1 using a center of symmetry method and find K1app = 37 ± 5 km/s. Accounting for the likely systematic errors affecting both K1 and K2 measurements, we conservatively derive 508 < K2 < 585 km/s and K1 < 37 km/s. This implies a massive white dwarf with M1 > 0.77M⊙. A non-degenerate mass donor, implying WZ Sge has not yet evolved through its minimum orbital period, is not ruled out by our observations. This would require an improved estimate of K1. Together with the measured phase offset between bright spot eclipse and inferior conjuction of the secondary star, we can bracket the allowed mass ratio (q = M2/M1) to lie between 0.040 and 0.073. This provides a firm upper limit to the mass of the secondary of M2 < 0.10M⊙. Subject headings: novae, cataclysmic variables — accretion, accretion discs — stars, individual (WZ Sge)