1994A&A...288L..45H

We compare the predicted ROSAT PSPC count rates and spectra for H-rich and He-rich white-dwarf LTE atmospheres with those for blackbodies. We show that a model-atmosphere fit to ROSAT PSPC data of the super-soft X-ray source 1E0056.8-7154 tends to give a higher effective temperature, smaller radius, and a much lower bolometric luminosity than a blackbody fit. This super-soft X-ray source has been identified with a planetary nebula in the Small Magellanic Cloud. For white-dwarf atmosphere models, the inferred bolometric luminosity is below the Eddington limit, and the derived radii are in better agreement with those expected for white dwarfs. Both H-burning and He-burning accreting white dwarfs in a binary are consistent with the ROSAT spectrum of 1E0056.8-7154. However, the spectrum, luminosity and radius are also consistent with an exceptionally hot single central star of a planetary nebula.