THE PROPERTIES OF BL LACERTAE OBJECTS

Based on the idea that the bulk velocity of the ejected plasma in BL L acertae objects increases with distance from the core, on the assumpti on of a relation between Doppler factor and frequency, and the known r adio Doppler factors, the properties of radio-selected BL Lac objects (RBLs) corrected for relativistic Doppler effect are shown to be consi stent with the properties of X-ray selected BL Lac objects (XBLs). Thi s is not the case for the observed (uncorrected) properties of RBLs. T his shows that RBLs and XBLs are probably similar objects seen from di fferent angles with respect to the direction of the jet.