HIGH-RESOLUTION UV SPECTROSCOPY OF 2 HOT (PRE-)WHITE-DWARFS WITH THE HUBBLE-SPACE-TELESCOPE - KPD-0005+5106 AND RXJ2117+3412

High resolution UV spectra of KPD 0005+5106 and RXJ2117+3412 have been obtained with the Goddard High Resolution Spectrograph aboard the Hub ble Space Telescope. The data were subject to a non-LTE model atmosphe re analysis. For KPD 0005+5106, the hottest known helium-rich white dw arf, the C and N abundances as well as an upper limit for O have been determined (log(C/He)=-3, log(N/He)=-6, log(O/He)<-5.5, by number), im proving results from previous studies. These abundances are severely a t odds with diffusion/radiative forces theory. They are possible primo rdial relies of their ancestral progenitor in the post-AGE phase. Besi des the photospheric lines with a velocity shift of +35 km s(-1) we fi nd a distinct interstellar/circumstellar absorption line system shifte d by -15 km s(-1). Difficulties are encountered with line profile fits to the spectrum of RXJ2117+3412, an extremely hot hydrogen-deficient (PG 1159-type) central star. Some of the strong C IV and O VI lines ar e too narrow to match the observations. Either the neglect of wind-eff ects in our models or the lack of reliable line broadening data may be responsible for this. A weak O V 1371 Angstrom line is detected which suggests, that the effective temperature is higher than thought befor e (T-eff=170 000 K). This means that RXJ2117+3412 is the hottest known PG 1159 star together with H1504+65. Interstellar and photospheric li ne spectra are clearly separated, with shifts of -19 and +23 km s(-1), respectively. Both stars exhibit a strong O VIII emission line near 2 977 Angstrom, which cannot be of photospheric origin but probably aris es from shock fronts along a stellar wind. The line widths are of the order of 150 km s(-1). In addition, KPD 0005+5106 shows another, equal ly strong emission feature near 2982 Angstrom which either stems from photospheric N V or from non-photospheric C V.