Environmental Properties of BL LAC Objects

In our ongoing effort to study the environmental properties of BL Lac objects we present images for 13 new fields and spectroscopic results for a subset of 5 fields. When combined with seven fields studied previously, we have evidence for companion galaxies and clusters around a total of 12 objects. Furthermore, the imaging results are confirmed by spectroscopic observations for nine cases. The distribution of richness of the BL Lac fields compared to that of FR I radio galaxies gives consistent results for the unification of the two classes. Virtually all BL Lacs with unknown redshift observed so far do not exhibit an obvious clustering of galaxies. This implies that these objects are either in clusters at z > 0.5 - 0.7 or they are truly isolated.