1996A&A...312L..13H

We present very deep, high resolution R band imaging of the BL Lac object PKS 0139-097 taken at the Nordic Optical Telescope under subarcsecond seeing conditions (FWHM∼0.8", limiting magnitude ∼26) as well as NIR observations in K' performed at the Calar Alto 2.2m telescope (FWHM∼1.0", limiting magnitude∼20.5). After applying modern deconvolution techniques such as the Lucy-Richardson algorithm and maximum entropy method to the images we found 4 features surrounding PKS 0139-097 within aradius of 3.1". Three features are visible in both colours, one feature in R only. Their apparent magnitudes are between 22-26 in R and 18.5-19.5 in K'. Assuming that the features are at the redshift of z=0.501 based on the spectroscopy of the BL Lac object presented by Stickel et al. (1993), their absolute magnitudes are approximately 1-2mag fainter than a L* galaxy in both colours. The colours of the two closest features are typical for Mg II absorbers at this redshift, whereas one feature is extremely red and may thus be either an early-type galaxy at high redshift or a red quasar. The feature, which is not visible in K' is blue and might be a late-type field galaxy at moderate redshift. Hence it is likely that at least one of lhe two closest features is responsible for the Mg II absorption seen in the spectrum of PKS 0139-097. PKS 0139-097 may be another BL Lac object affected by gravitational microlensing.