2004MNRAS.355.1031G

The use of observations of the hydrogen Balmer absorption series is a well-established method of determining the surface gravity and effective temperature (T_eff) of white dwarfs. In situations where the Balmer lines cannot be used it is possible to use the hydrogen Lyman series instead. However, previous studies by Barstow et al. have shown that for DAs (hydrogen-rich, no helium) hotter than ∼50000K the Lyman lines systematically yield higher values of T_effthan the Balmer lines. Analysis of optical and FUSE spectra of 16 DAO white dwarfs (hybrid hydrogen/helium) demonstrates that temperatures measured using the different data sets continue to diverge at even higher temperatures, and in three extreme cases T_effderived from the Lyman lines is in excess of 120000K. These discrepancies are observed to decrease at lower gravities and luminosities. Helium abundances were also determined from the strength of Heii lines in the two wavelength regimes. These agree well, even though the temperatures differ considerably, with the exception of two DAO+M dwarf binaries. For these, the change in the measured helium abundance may result from the different times at which the objects were observed in the two wavelength regimes.