This model is supported by the fact that almost all blazars are strong and rapidly variable $\gamma$–ray sources (von Montigny 1995). The beaming model implies the existence of a more numerous parent population of intrinsically identical objects but with their jet oriented at larger angles to our line–of–sight. In the current unified models for radio–loud AGN (Urry & Padovani 1995), BL Lac objects are unified with low luminosity core–dominated (F–R I) radio galaxies (RG) seen nearly along the jet axis, while the high luminosity lobe–dominated (F–R II) RGs represent the parents of FSRQs (Padovani & Urry 1990; Urry, Padovani & Stickel 1991). However, for potential problems in this simple unification, see Urry & Padovani (1995). For a direct test of the unification model, we need to compare orientation–independent properties of BL Lac objects with those of the parents, extended radio emission, host galaxies and environments.

Considerable amount of optical imaging exists for relatively nearby (z$\leq$0.5) BL Lac hosts (Abraham, McHardy & Crawford 1991; Stickel, Fried & Kühr 1993; Falomo 1996; Wurtz 1996; Falomo 1997b; Jannuzi, Yanny & Impey 1997). The host galaxies of nearby BL Lacs have turned out to be predominantly giant ellipticals with similar magnitude to F–R I 