SIMBAD references

Based on the idea that the bulk velocity of the ejected plasma in BL Lacertae objects increases with distance from the core, on the assumption of a relation between Doppler factor and frequency, and the known radio Doppler factors, the properties of radio-selected BL Lac objects (RBLs) corrected for relativistic Doppler effect are shown to be consistent with the properties of X-ray selected BL Lac objects (XBLs). This is not the case for the observed (uncorrected) properties of RBLs. This shows that RBLs and XBLs are probably similar objects seen from different angles with respect to the direction of the jet.