Does the unification of BL Lac and FR I radio galaxies require jet velocity structures?

We explore the viability of the unification of BL Lacs and FR 1 radio galaxies by comparing the core emission of radio galaxies with those of BL Lacs of similar extended radio power, taking advantage of the newly measured optical nuclear luminosity of FR I sources. The spectral properties of complete samples are also studied in the radio-optical luminosity plane: starting from the Spectral Energy Distribution (SED) of BL Lacs, we calculate the predicted luminosity of FR I nuclei in the frame of a simple one-zone model, by properly taking into account the relativistic transformations. We find that the bulk Lorentz factors required by the spread in the observed luminosities in all bands are significantly smaller than those implied by other, both observational and theoretical, considerations. This discrepancy is also reflected in the fact that FR I nuclei are overluminous by a factor of 10-10(4), with respect to the predictions, both in the radio and in the optical band.