(see below); and (7) the R–H colour gradient of the host. For comparison, in Table 4 we give optical and optical–NIR colours of elliptical galaxies from literature search. The average and median R–H host colours of our BL Lac sample, R–H = 2.2$\pm$0.5 and R–H = 2.3 are consistent with those found for local giant ellipticals (R–H = 2.5; see Table 4), considering the large variations both within the sample and in the previous optical determinations of the host parameters. The average R–H colour also agrees well with recent evolutionary population synthesis models, which predict for low redshift, solar metallicity ellipticals R–H$\sim$2.2 (Poggianti 1997; age 15 Gyr) and R–H$\sim$2.4 (Fioc & Rocca-Volmerange 1997; age 13 Gyr).

[lllllll]{}\
\
& M(H) & M(R) & Ref. & R-H & R-H & $\Delta$(R-H)/$\Delta$(log r)\
(1) & (2) & (3) & (4) & (5) & (6) & (7)\
\
PKS 0521–365 & -25.4 & -23.2 & F94 & 2.2 & 2.3 & -0.08\
& & -23.0 & W96 & 2.4 & &\
PKS 0548–322 & -25.7 & -24.2 & F95 & 1.5 & 2.1 & -0.06\
& & -23.2 & W96 & 2.5 & &\
PKS 1514–241 & -25.1 & -22.8 & B87 & 