The BL Lac Object PKS 0537-441： A Lens or Being Lensed?

We present and discuss deep high-resolution images taken with the ESO-NTT as well as spectra taken with the ESO-VLT in the field of the BL Lac object PKS 0537-441.We could neither detect the host galaxy nor a galaxy along the line-of-sight to the BL Lac as claimed from previous observations.We found 4 nearby companion galaxies which could be a splitted and magnified image of a distant background galaxy.Our VLT-spectra, however, rule out this possibility.All 4 companions have similar redshifts as the BL Lac and may (at least in part)trigger the enormous activity in this source.PKS 0537-441 might be located in a galaxy cluster as rich as Abell 1.Alternatively, the BL Lac could also be located in the outskirts of a galaxy cluster or even be projected on a large-scale structure as indicated by spectra of 30 galaxies in the field.Either lensing hypothesis (lens or being lensed)is considerably weakened by our observations.