1995A&A...294..183W

We present pointed observations of four hot DA white dwarfs with the ROSAT satellite. Measurements with the Position Sensitive Proportional Counter (PSPC) and the Wide Field Camera (WFC) were analyzed in the framework of pure hydrogen model atmospheres. The results fit well into the picture of previous studies that objects between about 20000 and 40000K have almost pure hydrogen atmospheres, while at higher temperatures additional absorbers are present in general. Of particular interest are PG0824+289, PG1657+344, and PG1658+441. The former is a close binary consisting of a DA white dwarf and a dwarf carbon star (dC). The effective temperature of 37000K, derived from the ROSAT observations, is very close to the value determined from IUE spectra, which - unlike optical spectra - are not contaminated by the "cool" dC. Therefore we conclude that the DA component of the system has a pure hydrogen atmosphere. The magnetic DA PG1658+441 has the highest mass (1.31M_☉) of all known single white dwarfs. Its X-ray flux can be explained by a pure hydrogen atmosphere. An optical spectrum of PG1657+344 reveals that this object is one of the few DA white dwarfs with T_eff>50000K. In this case the analysis of the ROSAT data shows clear evidence for absorbing material besides hydrogen.