Pulsations and Accretion Geometry in YY Draconis: A Study Based onHubble Space TelescopeObservations

We present 1 Å resolution fast ultraviolet spectroscopy of YY Dra, coordinated U, B, V, R , and I photometry, and Hα spectroscopy. The UV continuum is strongly pulsed: we found 16% semiamplitude pulses with period 264.7(1) s; there was no evidence for power at twice this period. The UBV pulses are in phase with the UV continuum pulsation. A color temperature of ≳10 5 K was found for the pulse spectrum, with a corresponding area ≲0.5% of the white dwarf surface. We refine the white dwarf spin period, 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 apparent when the data are folded on P spin . Faint, broad line wings extending to ±3000 km s -1 appear simultaneously with the continuum pulsation maxima. This implies that accretion was occurring onto both magnetic poles. Consequent constraints on the accretion geometry and white dwarf mass are derived, and a minimum white dwarf mass of 0.62 M ☉ is obtained. A color temperature T ≲ 30,000 K is derived for the unpulsed emission. We determine a precise orbital period, 0.16537398[17] days, based on an unambiguous 14 yr cycle count. We describe a simple phase-matching technique used in the period search. The UV continuum and UBVR photometry show a single-humped orbital modulation; the I -band light curve shows a double-humped "ellipsoidal" shape. The deeper I -band minimum occurs at inferior conjunction of the mass donor, which suggests that heating of the L 1 point dominates over gravity darkening. I -band light-curve modeling suggests i ≲ 50°. Orbital radial velocity variations of semiamplitude 91 ± 10 km s -1 were found in the C IV emission. Combining with the published value K s = 202 ± 3 km s -1 , this yields q = 0.45 ± 0.05. Assuming a Roche lobe-filling 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° ± 4°.