NEW BL LACERTAE OBJECTS FROM THE ROSAT ALL-SKY SURVEY

We have searched a sample of radio-loud X-ray sources from the ROSAT A ll-Sky Survey for BL Lacertae objects by means of field polarimetry an d follow-up spectroscopy. To this end we have studied a representative subset of 119 sources (hereafter ''XR sample'') from a correlation be tween the ROSAT catalogue and the Condon 6 cm radio survey of the nort hern sky. This ROSAT-Condon (hereafter ''RC'') correlation contains a total of 2127 sources. 84% of the XR sources are now identified or par tially observed and have so far yielded 14 confirmed BL Lac objects pl us 2 candidates, all of which are described in detail in this paper. T he observations leave us with tight constraints on the total number of XR BL Lacs that we estimate to be 20 +/- 2. The resulting BL Lac frac tion of (16.8 +/- 1.7)% is significantly higher than the (9.2 +/- 2.5) % expected for the RC correlation based on pre-ROSAT identifications. We assign the success of our search to the field polarimetry, which pr oved to be a reliable and complete identification method for BL Lacs. We regard the XR BL Lacs as particularly interesting because, rather t han keeping to the discrete areas in the alpha(OX)-alpha(RO) diagram c ustomary for the classical X-ray and radio selected objects, they are distributed in and between those regions. Due to their 'mixed' selecti on these objects might form a bridge between the two BL Lac types whic h may help to constrain unification models for BL Lacs.