Abstract
We present the results of a recent spectroscopic investigation of the cool DO white dwarf HD 149499 B in the EUV and FUV ranges. Observations were performed with the spectrograph of theEUVEsatellite and the Berkeley EUV/FUV spectrometer of theORFEUSspace experiment. The analysis of theORFEUSspectrum, performed with a grid of LTE model atmospheres, yielded the basic parametersTeff= 49500 ± 500 K and logg= 7.97 ± 0.08. This result is confirmed by theEUVEspectra. The photospheric hydrogen Lyman lines in the FUV spectrum indicate the presence of hydrogen: lognH/nHe= −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 performed by a comparison with NLTE atmospheres calculated for appropriate parameters. The interstellar hydrogen column towards the HD 149499 system amounts toNH= (7 ± 2) · 1018cm−2.
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