  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 ~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 ~
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.
