The optical-near-infrared colour of the host galaxies of BL Lacertae   objects
  We present H-band imaging of 23 low redshift (z < 0.3) BL Lac objects that were previously investigated by us in the R-band. For all the observed objects, the host galaxy is well represented by an elliptical model, with average scale length R(e) = 7.2 +- 3.6 kpc and average absolute magnitude M(H) = -25.9 +- 0.6. BL Lac hosts are therefore luminous elliptical galaxies indistinguishable from those of low redshift radio galaxies and inactive ellipticals. The best-fit H-band Kormendy relation of mu(e) = 3.7 log r(e) (kpc) + 14.8 mag arcsec^-2 agrees with that obtained for normal ellipticals and radio galaxies. This similarity indicates that all massive elliptical galaxies can experience nuclear activity without significant perturbation of their global structure. The new observations are combined with literature data of 18 objects, and this extended sample is used to investigate the optical-NIR colour and colour gradient properties of the host galaxies of BL Lacs and to perform a direct comparison with other elliptical galaxies with and without nuclear activity. The optical/near-infrared colour (R-H = 2.2 +- 0.4) and colour gradient (Delta(R-H)/Delta(log r) = -0.38 +- 0.60) distributions of the BL Lac hosts are much wider than those for normal ellipticals with old stellar populations, and many BL Lacs have bluer hosts and/or steeper colour gradients than those in normal ellipticals. The blue colours are most likely caused by a young stellar population, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. However, the lack of obvious signs of interaction may require a significant time delay between the event with associated SF episodes and the onset of nuclear activity. 
J.K. Kotilainen (Tuorla Observatory), R. Falomo (Osservatorio di   Padova)