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 prev iously, 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 richne ss of the BL Lac fields compared to that of FR I radio galaxies gives consistent results for the unification of the two classes. Virtually a ll BL Lacs with unknown redshift observed so far do not exhibit an obv ious clustering of galaxies, This implies that these objects are eithe r in clusters at z>0.5-0.7 or they are truly isolated. (C) 1995 Americ an Astronomical Society.