ORFEUS OBSERVATIONS OF THE DO WHITE-DWARF HD-149499-B

We present an analysis of a high-resolution FUV spectrum (lambda = 912 Angstrom ... 1170 Angstrom) of the cool DO white dwarf HD 149499 B. O bservations were performed with the Berkeley EUV/FUV spectrometer of t he ORFEUS experiment. The analysis, performed with a grid of LTE model atmospheres, yielded the basic parameters T-eff = 49500 +/- 500 K and log g = 7.97 +/- 0.08. The photospheric hydrogen Lyman lines in the F UV spectrum indicate the presence of hydrogen: log n(H)/n(e), = -0.65 +/- 0.12. The implications of this finding for the spectral evolution of white dwarfs are discussed. A check of the LTE assumption was perfo rmed by a comparison with NLTE atmospheres calculated for appropriate parameters. The interstellar components of the hydrogen Lyman lines ar e used for an estimate of the hydrogen column towards the HD 149499 sy stem: N-H approximate to 1.10(19)cm(-2).