Far-ultraviolet spectroscopy of pg 1159 stars with the Hopkins ultraviolettelescope

We present for the first time systematic far-ultraviolet (FUV) spectroscopy that extends down to the hydrogen Lyman edge for five representative PG 11 59 stars (H1504+65, K1-16, PG 1424+535, PG 1707+427, and PG 1159-035). In a ddition, we have observed the hottest known DO white dwarf (KPD 0005+5106) and the hot hydrogen-rich central star of the planetary nebula Sh 2-216. Th e medium-resolution spectra were taken with the Hopkins Ultraviolet Telesco pe (HUT) during the Astro-2 mission in 1995. We have performed line identif ications and non-LTE model atmosphere fits. The results confirm earlier ana lyses of optical and UV data and complement recent Hubble Space Telescope G oddard High-Resolution Spectrograph UV spectroscopy. The following conclusi ons are drawn from the HUT data. The absence of absorption lines from the H e II Balmer series finally proves the helium deficiency in H1504+65 and con firms its nature as a naked C/O stellar core. The detection of a P Cygni pr ofile from the O VI resonance line in the spectrum of the central star of K 1-16 allows a first reliable determination of the mass-loss rate from a PG 1159 star. It suggests that the wind is radiation driven. The spectra of th e pulsator/nonpulsator pair PG 1707 + 427 and PG 1424 + 535 reveal differen ces, which could not be detected with optical spectroscopy, in the photosph eric parameters. In particular, we announce the detection in the pulsator P G 1707 + 427 of nitrogen, which is a remarkable phenomenon among PG 1159 st ars. Also, in the prototype itself we find hints for photospheric nitrogen. The hot DO KPD 0005 + 5106 exhibits H-2 absorption lines similar to those found in the spectrum of the central star Sh 2-216. This may signify the pr esence of an old planetary nebula surrounding the DO star.