The stellar content of low redshift BL Lacertae host galaxies from multicolour imaging

Context. We present B-band imaging of 18 low redshift ($z \leq 0.3$) BL Lac objects for which their host galaxies were previously resolved in the R-band and the near-infrared H-band. For a subset of the objects, U- and V-band imaging also is presented. Aims. These multiwavelength data are used to investigate the blue–red–near-infrared colours and the colour gradients of the host galaxies of BL Lacs in comparison with other elliptical galaxies with and without nuclear activity. Methods. For all the BL Lacs observed in the B- and V-bands, and all objects at $z