uncertainty. The lack of direct comparison sample of F-R I RGs, however, prevents us from discussing the issue of the parent population of BL Lac objects further.

Since BL Lacs share many properties (variability and polarization) with FSRQs, it is interesting to compare the host properties of these two types of blazars. Recently, KFS98 found for clearly resolved FSRQ hosts at z$\sim$0.7 M(H) = –27.8$\pm$0.3, while the value adding their marginally resolved hosts is M(H) = –26.7$\pm$1.2 (Table 5). Because not all FSRQs were resolved, it is possible that the average host luminosity of FSRQs is even fainter. Moreover, given the large difference of redshift between the BL Lac and FSRQ samples, it is also possible that the $\sim$0.5 mag difference in host luminosity is due to cosmological evolution in the stellar population of an elliptical galaxy that makes it fade by $\sim$0.8 mag between z$\sim$1 and z$\sim$0 (models by Bressan 1994). Indeed, there is increasing observational evidence for significant evolution in the colours and luminosity of early–type galaxies (Aragon-Salamanca 1998 and references therein). The present data suggests that BL Lac hosts are less luminous than those of FSRQs but, given the limits described above, additional homogeneous observations of BL 