HOT WHITE-DWARFS IN THE EXTREME-ULTRAVIOLET EXPLORER SURVEY .1. PROPERTIES OF A SOUTHERN-HEMISPHERE SAMPLE

We present optical spectroscopy of the Balmer line series (H beta thro ugh H8) of a sample of 18 hot white dwarfs obtained at the Mount Strom lo and Siding Spring Observatories; the stars are a subset of the whit e dwarfs detected in the Extreme Ultraviolet Explorer (EUVE) all-sky s urvey. We analyze the Balmer line profiles conjointly with EUV photome tric measurements and provide new effective temperature and surface gr avity determinations. We also provide determinations of the neutral hy drogen column density in the interstellar medium toward these lines of Sight. The spectral and photometric properties of most objects in our sample are consistent with emission from pure hydrogen atmospheres. H owever, we find evidence for the presence of trace absorbers in the ph otospheres of five of IS objects from the sample - EUVE J0230-479, EUV E J0809-729, EUVE J1546-370, EUVE J2004-560, and EUVE J2324-547. The s tar EUVE J0230-479 figures among the group of DA white dwarfs with the highest level of metallicity, while EUVE J0809-729 is one of the cool est DA stars (approximate to 30,000 K) to show evidence of trace eleme nts. We also report the discovery of three new massive white dwarf sta rs-EUVE J1546-367 (M approximate to 1.0 M.), EUVE J1727-360 (M approxi mate to 1.2 M.), and EUVE J1746-706 (M approximate to 1.4 M.); excludi ng the two high-mass (M > 1.1 M.) objects from our sample, we obtain a n average white dwarf mass of 0.64 M..