We have next computed radial R–H colour profiles for six of the BL Lacs (Fig. 3). From these profiles we have computed the R–H colour gradients of the host galaxies as reported in Table 3, column (7). With the exception of PKS 2254+074 (see Appendix), all colour profiles show a modest colour gradient (average $\Delta$(R–H)/$\Delta$(log r) = –0.09$\pm$0.04) that makes the galaxies bluer in the outer regions. The sign and amplitude of the color gradient is similar to that exhibited by normal non-active ellipticals ($\Delta$(V–K)/$\Delta$(log r) = –0.16$\pm$0.18; Peletier, Valentijn & Jameson 1990; 12 ellipticals); $\Delta$(V–K)/$\Delta$(log r) = –0.26$\pm$0.15; Schombert 1993; 16); the R–H gradient tends to be smaller than that of V–K, probably due to the smaller wavelength baseline. Finally, we show in Fig. 4 the colour–magnitude diagram of the BL Lac host galaxies, compared with those of elliptical and S0 galaxies in the Virgo and Coma clusters (Bower, Lucey & Ellis 1992a,b). With the exceptions of PKS 0548–322 and PKS 2254+074 (see Appendix for details), the BL Lac hosts follow reasonably well the established relation for elliptical galaxies.

There is increasing evidence that the photometric properties of elliptical galaxies (at least of those in nearby clusters) can be 