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Observations of hot DA white dwarfs in the EUV/soft X-ray range have revealed that, in a majority of cases, the detected flux is less than that expected from pure hydrogen atmospheres. This implies an extra opacity source which must be due to the presence of small traces of heavier elements. These elements are generally not spectroscopically detected in hot DA white dwarfs, but the large sensitivity of the EUV/soft X-ray broad-band flux to the presence of extra absorbers can be used with profit to Infer their abundances. For simplicity, it has been assumed that only helium provides the required opacity source in the majority of the analyses carried out so far. In this context, Vennes et al. (1988a) have recently reviewed in details the mechanisms that could be responsible for the presence of small traces of helium in the atmospheres of hot DA white dwarfs. They favor a model in which these stars are interpreted as stratified objects with an outer layer of hydrogen which is sufficiently thick that radiation in the visible escapes only from H-rich regions, and yet sufficiently thin that the EUV/soft X-ray radiation escapes from deeper layers, polluted by the tail of the helium distribution which extends upwards. This model accounts naturally for the positive correlation observed between the inferred helium abundance and the effective temperature in hot DA stars studied at short wavelengths. If the model is correct, hot DA white dwarfs as a class must have very thin outer hydrogen layers with estimated masses in the range–13 > log q(H) = log (M(H)/M) > –15.
