A STUDY OF THE ABSORPTION-LINES FROM THE DONOR STAR IN THE EXOTIC CATACLYSMIC VARIABLE AE AQUARII

We estimate the fundamental system parameters (e.g. mass ratio, inclin ation, stellar masses) of the unusual cataclysmic variable AE Aquarii by characterizing the behaviour of the mass-donor star. Our analysis f ocuses on the absorption lines from the donor star, avoiding biases in duced by use of the poorly understood emission lines. Using simultaneo us high-speed photometry and high-resolution Ha spectroscopy, we obser ved weak absorption lines in AE Aquarii's spectrum. By modelling these lines we estimate the spectral type of the mass-donor star (K3-K5), i ts orbital and rotation velocities (K-1 similar to 138 +/- 6 km s(-1), K-2 = 157.9 +/- 0.8 km s(-1); V-rot sin i = 85-108 km s(-1)), and its contribution to the total flux of the system (69-94 per cent). From K -2 and V-rot sin i we derive the mass ratio 0.58 < q = M(2)/M(1) < 0.8 9, the range depending upon the spectral type and limb darkening. By u sing K-2, and the amplitude of the pulse arrival-time delay curve of E racleous et al., we estimate the mass ratio of the binary system (q = 0.646 +/- 0.013). We also observe variations in the absorption-line fl ux deficit, and, if interpreted as ellipsoidal variations, their ampli tudes suggest an inclination i=49 degrees+/-15 degrees. However, the d eviations from the expected ellipsoidal variations are significant, su ggesting a non-uniform distribution of absorption-line strength across the donor star (perhaps indicative of star-spot regions). Realistic p robability distributions for the inclination and the stellar masses ar e derived using a Monte Carlo simulator assuming M(1) < 1.4 M., but wi th no assumptions about the mass of the donor star. We find M(1) = 0.8 9 +/- 0.23 M., M(2) = 0.57 +/- 0.15 M., and further constrain i to 55 degrees +/- 7 degrees. We also report the serendipitous discovery of a (previously unknown) eclipsing variable star near AE Aquarii.