PULSATIONS AND ACCRETION GEOMETRY IN YY DRACONIS - A STUDY BASED ON HUBBLE-SPACE-TELESCOPE OBSERVATIONS

We present 1 Angstrom resolution fast ultraviolet spectroscopy of YY D ra, coordinated U, B, V, R, and I photometry, and H alpha spectroscopy . The UV continuum is strongly pulsed: we found 16% semiamplitude puls es with period 264.7(1) s; there was no evidence for power at twice th is period. The UBV pulses are in phase with the UV continuum pulsation . A color temperature of greater than or similar to 10(5) K was found for the pulse spectrum, with a corresponding area less than or similar to 0.5% of the white dwarf surface. We refine the white dwarf spin pe riod, obtaining P-spin = 529.31 +/- 0.02 s. Optical pulses at 273(1) s were also detected; we attribute these to reprocessing in structures fixed in the orbital frame. Variations in the C IV line profile are ap parent when the data are folded on P-spin. Faint, broad line wings ext ending to +/- 3000 km s(-1) appear simultaneously with the continuum p ulsation maxima. This implies that accretion was occurring onto both m agnetic poles. Consequent constraints on the accretion geometry and wh ite dwarf mass are derived, and a minimum white dwarf mass of 0.62 M. is obtained. A color temperature T less than or similar to 30,000 K is derived for the unpulsed emission. We determine a precise orbital per iod, 0.16537398[17] days, based on an unambiguous 14 yr cycle count. W e describe a simple phase-matching technique used in the period search . The UV continuum and UBVR photometry show a single-humped orbital mo dulation; the I-band light curve shows a double-humped ''ellipsoidal'' shape. The deeper I-band minimum occurs at inferior conjunction of th e mass donor, which suggests that heating of the L(1) point dominates over gravity darkening. I-band lightcurve modeling suggests i less tha n or similar to 50 degrees. Orbital radial velocity variations of semi amplitude 91 +/- 10 km s(-1) were found in the C IV emission. Combinin g with the published value K-s = 202 +/- 3 km s(-1) this yields q = 0. 45 +/- 0.05. Assuming a Roche lobe-filing main-sequence mass donor and adopting the empirical ZAMS mass-radius relation, we obtain M(s) = 0. 375 +/- 0.014 M., M(wd) = 0.83 +/- 0.10 M., and i = 45 degrees +/- 4 d egrees.