BL Lacertae objects in the ROSAT All-Sky Survey: new objects and comparison of different search techniques.

We report the discovery of 34 BL Lac objects identified during the correlation of the ROSAT All-Sky Survey (RASS) with the Hamburg Quasar Survey (HQS). We find that most of the newly identified sources have high ratios of X-ray to optical flux, log(f_X_/f_B_)>1.3, and use this criterion as an efficient, radio-independent means for selecting candidate BL Lac objects. Combining these new sources with 43 previously known BL Lac objects in the ~6000deg^2^ surveyed area, allowed us to construct a new sample complete down to f_X_(0.1-2.4keV)=~5.5x10^-12^ergs/cm^2^/s. Using this sample, we compare the efficiency and selection effects inherent in two different search techniques: The first characterized by the selection of high log(f_X_/f_B_)objects and second by selection of objects with particular X-ray/radio/optical colour indices. The logS-logN diagram of the sample is consistent with results found in previous X-ray surveys, although it shows indications of more X-ray faint BL Lacs than previously thought. We expect ~200 additional BL Lac objects among the remaining high log(f_X_/f_B_) BL Lac candidates in the surveyed area.