We investigate why BL Lacertae objects ( BL Lacs ) have values of the Ca H & K break ( a stellar absorption feature ) lower than low-power radio galaxies and if its use is justified to separate the two classes .
For this purpose we relate this parameter to the radio and optical core emissions , as well as to the X-ray powers , for a sample of \sim 90 radio sources .
We find that the Ca H & K break value decreases with increasing jet powers , and that it also anti-correlates with the radio core dominance parameter but not with extended radio emission .
Based on this we conclude that the Ca H & K break value of BL Lacs and radio galaxies is a suitable indicator of orientation .
From the luminosity ratios between objects with low and high Ca H & K break values we constrain the average Lorentz factors for BL Lacs and low-power radio galaxies in the radio and X-ray band to \Gamma \sim 2 - 4 and derive average viewing angles for the galaxies .
Our values are in agreement with results from independent methods .
We find that the correlations between Ca H & K break and radio core and X-ray luminosity hold independently for low- ( LBL ) and high-energy peaked BL Lacs ( HBL ) .
We derive average viewing angles for their parent populations , which turn out to be similar to the ones for our entire sample , and compare for the first time the luminosities of LBL and HBL at different orientations .