We present FUSE observations of the hot white dwarfs in the post-common envelope binaries Feige 24 , EUVE J0720 - 317 , BPM 6502 , and EUVE J2013 + 400 .
The spectra show numerous photospheric absorption lines which trace the white dwarf orbital motion .
We report the detection of C iii , O vi , P v , and Si iv in the spectra of Feige 24 , EUVE J0720 - 317 and EUVE J2013 + 400 , and the detection of C iii , N ii , Si iii , Si iv , and Fe iii in the spectra of BPM 6502 .
Abundance measurements support the possibility that white dwarfs in post-common envelope binaries accrete material from the secondary star wind .
The FUSE observations of BPM 6502 and EUVE J2013 + 400 cover a complete binary orbit .
We used the FUSE spectra to measure the radial velocities traced by the white dwarf in the four binaries , where the zero-point velocity were fixed using the ISM velocities in the line of sight of the stellar systems .
For BPM 6502 we determined a white dwarf velocity semi-amplitude of K _ { WD } = 18.6 \pm 0.5 km s ^ { -1 } , and with the velocity semi-amplitude of the red dwarf companion ( K _ { RD } = 75.2 \pm 3.1 km s ^ { -1 } ) , we estimate the mass ratio to be q = 0.25 \pm 0.01 .
Adopting a spectroscopic mass determination for the white dwarf , we infer a low secondary mass of M _ { RD } = 0.14 \pm 0.01 M _ { \odot } .
For EUVE J2013 + 400 we determine a white dwarf velocity semi-amplitude of K _ { WD } = 36.7 \pm 0.7 km s ^ { -1 } .
The FUSE observations of EUVE J0720 - 317 cover approximately 30 % of the binary period and combined with the HST GHRS measurements ( Vennes et al .
1999 , ApJ 523 , 386 ) , we update the binary properties . FUSE observations of Feige 24 cover approximately 60 % of the orbit and we combine this data set with HST STIS ( Vennes et al .
2000 , ApJ , 544 , 423 ) data to update the binary properties .