New BL Lacertae objects from the ROSAT All-Sky Survey.

We have searched a sample of radio-loud X-ray sources from the ROSAT All-Sky Survey for BL Lacertae objects by means of field polarimetry and follow-up spectroscopy. To this end we have studied a representative subset of 119 sources (hereafter "XR sample") from a correlation between the ROSAT catalogue and the Condon 6cm radio survey of the northern sky. This ROSAT-Condon (hereafter "RC") correlation contains a total of 2127 sources. 84% of the XR sources are now identified or partially observed and have so far yielded 14 confirmed BL Lac objects plus 2 candidates, all of which are described in detail in this paper. The 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 fraction 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 proved to be a reliable and complete identification method for BL Lacs. We regard the XR BL Lacs as particularly interesting because, rather than keeping to the discrete areas in the α_OX_-α_RO_ diagram customary for the classical X-ray and radio selected objects, they are distributed in and between those regions. Due to their `mixed' selection these objects might form a bridge between the two BL Lac types which may help to constrain unification models for BL Lacs.