Abstract
We present UBV-band imaging of 18 low redshift (z ≤ 0.3) BL Lac objects. These data are used to investigate the blue-red-near-infrared colours and colour gradients of the host galaxies. The distributions of the integrated blue/near-infrared colour and colour gradient of the BL Lac hosts are much wider than those for inactive ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour gradients than those in inactive ellipticals. The blue colours are likely caused by a young stellar population component, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. This result is supported by stellar population modelling, indicating a presence of young/intermediate age populations in the majority of the sample.
