Emission from the secondary star in the old cataclysmic variable WZ Sagittae

We present the first detection of the mass donor star in the cataclysmic variable WZ Sagittae. 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(-1) and an apparent velocity amplitude of K-2' = 493 +/- 10 km s(-1). Doppler tomography reveals a highly asymmetric accretion disk including a significant bright spot contribution 20 days into the outburst. We estimate the apparent primary radial velocity using the H alpha tomogram and find K-1' = 37 +/- 5 km s(-1). Accounting for the likely systematic errors affecting both K-1 and K-2 and measurements, we conservatively derive 493 km s(-1) < K-2 <585 km s(-1) and K-1 < 37 km s(-1). The measured phase offset between bright spot eclipse and inferior conjunction of the secondary star brackets the allowed mass ratio (q = M-2/M-1) to lie between 0.040 and 0.075. These constraints imply a white dwarf with M-1 > 0.70 M-. and a low-mass secondary with M-2 < 0.11 M-.. A nondegenerate mass donor, implying WZ Sge has not yet evolved through its minimum period, is therefore not ruled out by our observations. This would require an improved estimate of K-1.