Hot white dwarfs in the extreme-ultraviolet explorer survey. IV. DA white dwarfs with bright companions

We present an analysis of optical, ultraviolet, and X-ray spectral properti es of a sample of 13 hot hydrogen-rich (DA) white dwarfs, each paired with a luminous unresolved companion. Using low-dispersion International Ultravi olet Explorer spectra, ROSAT photometry, and Extreme-Ultraviolet Explorer p hotometry and spectroscopy, we estimate the effective temperature, mass, an d distance of the white dwarfs. Additionally, we examine the question of th eir atmospheric composition. We establish orbital properties for most binar ies by means of high-dispersion optical spectroscopy obtained with the Hami lton echelle spectrograph at Lick Observatory; the same data help uncover e vidence of activity in some of the secondary stars that is also notable in ROSAT X-ray measurements. In particular, we find high-amplitude (>20 km s(- 1)) velocity variations in only two stars (HD 33959C and HR 8210), low-ampl itude variations in four additional objects (HD 18131, HR 1608, theta Hya, and BD +27 degrees 1888), and no variations (<2 km s(-1)) in the remainder. We have observed Ca H and K in emission in four (BD +08 degrees 102, HD 18 131, HR 1608, and EUVE J0702 + 129) of the six objects that were also detec ted in the 0.52-2.01 keV ROSAT PSPC band, while the source of the hard X-ra y emission in HD 33959C remains unknown; other investigators have noted som e evidence of activity in the remaining 0.52-2.01 keV detection, HD 217411. Properties of the white dwarfs are also investigated; EUV spectroscopy sho ws the effect of a low heavy element abundance in the atmosphere of the whi te dwarf in HD 33959C and of a high heavy element abundance in HD 223816; m easurements of all other objects are apparently consistent with emission fr om pure-hydrogen atmospheres. However, current data do not constrain well t he white dwarf parameters, and, to remedy the situation, we propose to obta in spectroscopy of the complete H Lyman line series.