1996A&A...310..384G

We have discovered four AGN in the ROSAT all-sky-survey data with very steep X-ray spectra. We apply several models to these X-ray spectra with emphasis on warm absorber models which give an adequate description of the data. We report on the follow-up optical and radio observations which allow the identification of three of these objects as Narrow Line Seyfert 1 galaxies, and the fourth as BL Lac object. We have measured small-FWHM Hβ lines, strong FeII emission and weak [OIII] emission in the three Narrow Line Seyfert 1 galaxies, in line with known correlations with respect to the steepness of the X-ray spectra. We have discovered strong optical variability in the BL Lac object and two of the Seyfert galaxies using photographic plates of the Sonneberg Observatory field patrol. We finally discuss the statistical implications of our search algorithm on the expected number density of soft X-ray selected AGN and conclude that up to 30% of X-ray selected AGN might have supersoft X-ray spectra.