We present a comprehensive study of Far Ultraviolet Spectroscopic Explorer ( FUSE ) spectra ( 912–1190 Å ) of two members of the PG 1159 spectral class , which consists of hydrogen-deficient ( pre- ) white dwarfs with effective temperatures in the range T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 75 000–200 000 K. As two representatives of the cooler objects , we have selected PG 1707+427 ( T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 85 000 K ) and PG 1424+535 ( T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 110 000 K ) , complementing a previous study of the hotter prototype PG 1159 - 035 ( T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 140 000 K ) .
The helium-dominated atmospheres are strongly enriched in carbon and oxygen , therefore , their spectra are dominated by lines from C iii-iv and O iii-vi , many of which were never observed before in hot stars .
In addition , lines of many other metals ( N , F , Ne , Si , P , S , Ar , Fe ) are detectable , demonstrating that observations in this spectral region are most rewarding when compared to the near-ultraviolet and optical wavelength bands .
We perform abundance analyses of these species and derive upper limits for several undetected light and heavy metals including iron-group and trans-iron elements .
The results are compared to predictions of stellar evolution models for neutron-capture nucleosynthesis and good agreement is found .