explained in terms of a single burst of star formation at high redshift, followed by passive stellar evolution (Stanford, Eisenhardt & Dickinson 1998 and references therein). The similarity of the colours and colour gradients of BL Lac hosts to those of normal elliptical galaxies then indicates that the nuclear activity has only marginal effect, if any, on the overall properties of the host galaxy. For example, galaxy interactions and mergers, often invoked to explain the BL Lac phenomenon and AGN in general (Heckman 1990), do not seem to induce strong recent star formation in the BL Lac hosts. This is perhaps due to a very low gas density in the host and/or in the interacting galaxy. Alternatively, it may be that the timescales of the star formation episodes and nuclear activity are different.

Table 5 presents a comparison of the H–band absolute magnitudes of the BL Lac hosts with the average values of relevant samples of blazars and RGs from previous optical and NIR studies in the literature. Column (1) gives the sample; (2) the filter; (3) the number of objects in the sample; (4) the average redshift of the sample; and (5) and (6) the average H band nuclear 