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We have initiated a systematic study of the environmental properties of a sample of BL Lacertae objects (BL Lacs) using the NTT at La Silla, Chile. Here we report on the FOCAS analysis of the large-scale images ($7.5 \times 7.5$ arcmin$^{2}$) of seventeen objects. For seven BL Lacs we obtained spectra of at least one galaxy in the field.
We find that BL Lac objects are located in regions of higher-than-average galaxy density. For five sources we note the BL Lac has a very close ($\lsim$ 5 arcsec) companion galaxy, which is, perhaps, in the process of interacting. The small groups or clusters of galaxies possibly associated with the BL Lacs are rarely richer than Abell richness class 0. The BL Lac host galaxies are probably the dominant members of these clusters. Spectroscopy confirms the physical association between the BL Lac and galaxies in the field in six of seven cases observed.
A comparison of the environmental properties of BL Lacs with those of F-R I and II radio galaxies, the proposed parent population, indicates that BL Lac environments are similar to those of both types of radio galaxies. However, the analysis of a larger sample of BL Lac objects and F-R I and II radio galaxies is necessary before firm conclusions can be reached.
