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We present high-resolution imaging and low-resolution spectroscopy of the BLLac object PKS 0537-441 (z = 0.893) and its environment. The observations weredesigned to clarify, whether the properties of PKS 0537-441 are affected bygravitational microlensing, or whether PKS 0537-441 and its environment act asa lensing system itself. Our observations show that neither case seems to belikely. We did not find a galaxy along the line-of-sight to the BL Lac asclaimed previously, our spectroscopy shows that none of the four closestcompanion galaxies is at high redshift. Two of the four nearby companiongalaxies to PKS 0537-441 are within 200 km/s of the systemic velocity of the BLLac (z = 0.892 and 0.895, respectively). The third companion galaxy is athigher redshift (z = 0.947). The fourth companion galaxy shows evidence of MgII absorption redwards of its systemic velocity and is perhaps a mini lowionization BAL QSO at z = 0.885. If the latter can be confirmed, PKS 0537-441is the first BL Lacertae object being a member of a binary Quasar. We alsodetected extended [O II] emission in the off-nuclear spectrum of PKS 05371-441,which is most likely due to photoionization from the active nucleus.Alternatively, the extended [O II] emission is due to jet-cloud interactionwith the counterjet of PKS 0537-441. Our clustering analysis indicates that PKS0537-441 is located in a cluster environment as rich as Abell type 0-1. This issupported by the detection of four galaxies in the field with similar redshiftsas the BL Lac (Delta z < 0.002). We found serendipitously even more galaxies atsomewhat higher redshifts (z = 0.9-1). Thus, PKS0537-441 might be located infront of a galaxy cluster at somewhat higher redshift or even be part of alarge-scale structure with an extension towards the BL Lac.Comment: 13 pages, 9 figures, accepted for publication in A&

PKS 0537-441: Extended [O II] emission and a binary QSO?