Optical imaging of BL Lac host galaxies.

We have imaged 23 BL Lacertae objects (BL Lacs) with the William Herschel Telescope in a search for underlying host galaxies. Fourteen of these objects are resolved in our survey. We have determined the morphology of three of these galaxies for the first time, and have confirmed the morphology of an additional three. All the observed underlying hosts are centred on their bright cores, and two of the newly classified hosts are disc systems. These results are in disagreement with the predictions of standard beaming and lensing models for these objects. We have calculated absolute magnitudes for observed hosts with known redshifts and find them to be consistent with the results obtained by previous authors, despite the unexpected morphology exhibited by some of the underlying hosts.