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The only stars other than white dwarfs whose photospheric extreme ultraviolet radiation has been detected are e and β CMa. It is therefore of considerable theoretical interest to compare the EUVE observations of these two giant B stars to predicted spectra. However, both LTE and non-LTE very sophisticated line blanketed model atmospheres fail to match the observed flux. This failure leaves the stellar photosphere theory, even for seemingly “simple” objects as normal B giants were believed to be, in a rather dubious position. This paper briefly summarizes possible reasons for the failure of existing models to describe the EUVE observations of hot stars. In particular, we discuss the effects of uncertainties in the line blanketing, and the effects of the photosphere-wind interaction.

Although NLTE model atmospheres have been shown to resolve most of the equivalent width (EW) discrepancies for blue He I lines (Auer and Mihalas 1972, 1973), Wolff and Heasley (1984, 1985) have demonstrated that discrepancies remain for the leading members of the singlet/triplet 2P — nD series, viz. λ6678 and λ5876. These two lines are the strongest nonresonance He I transitions and are important because they respond to thermal changes in the superficial atmosphere (τ ∼ 10-3) of early B stars. In order to understand the observed rapid variations of the λ6678 line in mild Be stars, we undertook a survey of EWs of λ6678 and λ4388, namely the first and third member of the same series. These two lines have a log gf ratio of 15 but have similar EWs in B star spectra. Our new observations confirm the red line discrepancy noted by WH85 and point to additional EW differences among various groups of B stars not noted hitherto.

On the basis of IUE data, the specific absorption features in the spectra of Ap-Si Stars are displayed in the 1250-1850 A range and identified when possible. The contribution of the Si II multiplets is calculated in LTE for a typical overabundance x 102 of silicon. It accounts for only a small part of the observed absorption. Common unidentified structures, smaller than the 1400 A depression are pointed out.