1994A&A...286..381B

We detected 10 new BL Lac objects in the course of an ongoing identification program of ROSAT All Sky Survey (RASS) sources. They were found by follow-up spectroscopy of AGN candidates identified on digitized objective prism and direct photographic plates. All optical spectra show strong nonthermal continua with no detectable emission lines. In two cases, absorption lines enabled us to determine a redshift. For the majority of the sources the X-ray to optical flux ratio is extremely high, exceeding in many cases the highest value found for emission line AGN. This suggests to use this ratio as an efficient selection criterion for BL Lac objects in the RASS. As radio measurements are not required, this criterion is specially appropriate to select radio faint BL Lac objects. For most of the objects studied sufficient X-ray counts were gathered in the RASS to allow a detailed spectral analysis. Basically no differences were found between the properties of the new sources and those of already known BL Lac objects. The only exception is RX J10313+5053 which shows a a significant flattening of the soft X-ray spectrum beyond 0.7keV.