SIMBAD references

87 pointed PSPC (Position Sensitive Proportional Counter) observations of hot DA white dwarfs, available in the ROSAT public archive until the end of 1994, have been analyzed together with Wide Field Camera (WFC) and EXOSAT data. From the 57 objects with a significant count rate half of the objects have a temperature below 38,000K and a virtually pure hydrogen atmosphere. At higher temperatures most (75%) objects emit a lower X-ray flux than predicted by hydrogen model atmospheres and must therefore contain heavier elements as absorbers. This is probably due to radiative acceleration which prevents these elements from sinking inwards. We also confirmed our previous result that the PSPC detector is a factor of two more sensitive to photons with energies below about 0.2keV.