Blue colours of BL Lac host galaxies
  Near-infrared and optical imaging of BL Lac host galaxies is used to investigate their colour properties. We find that the R-H colour and colour gradient distributions of the BL Lac hosts are much wider than those for normal ellipticals, and many objects have very blue hosts and/or steep colour gradients. The blue colours are most likely caused by recent star formation. The lack of obvious signs of interaction may, however, require a significant time delay between the interaction event with associated star formation episodes and the onset of the nuclear activity. 
J.K. Kotilainen (1), R. Falomo (2) ((1) Tuorla Observatory, (2) INAF -   Padova Observatory)