THE HOT WHITE-DWARF COMPANIONS OF HR-1608, HR-8210, AND HD-15638

We have obtained low-dispersion IUE spectra of the late-type stars HD 15638 (F3 V), HR 1608 (=63 Eridani, K0 IV), and HR 8210 (A8m). Each of these stars had been detected as a strong EUV source with the Wide Field Camera (WFC) aboard the ROSAT satellite. The short-wavelength IUE spectrum of each star reveals the presence of a hot white-dwarf companion. We have fit the Lyman alpha profile and ultraviolet continuum of each white dwarf using pure hydrogen models. The excellent fit of the data to the models provides confirmation of the Finley and Koester absolute calibration of the SWP camera of IUE. The ultraviolet data alone are insufficient to constrain the model gravity, but an additional constraint is provided by the photometric distance to the late-type primary. We derive T(eff)=50,000(-9500/+2000) K, log g=8.15(+0.15/-0.80) for the white-dwarf companion to HD 15638, T(eff)=26,000 +/- 1500 K, log g=7.5 +/- 0. 5 for the companion to HR 1608, and T(eff)=3 5,500 +/- 1500 K and log g=9.0(+0.15/-0.3) for the companion to HR 8210. The most interesting of the three white dwarfs is the companion to HR 8210 for which our results imply a mass of 1.15(+0.05/-0.15) M.. This result is in good agreement with the lower limit on the mass derived from the spectroscopic orbit (M> 1.1 M.), provided that the inclination is close to 90-degrees. HR 8210 thus shows similarities to the Sirius system, although with a cooler primary (7700 K vs 10,000 K), a hotter white dwarf (35,500 K vs 25,000 K), and a 100 times smaller component separation (3 X 10(7) km vs 3 X 10(9) km). We discuss the importance of future observations of HR 8210 for the study of high-mass white dwarfs, and for scenarios of binary-star evolution.