2012A&A...547A...1N

We present the results of deep I-band imaging of two BL Lacerate objects, RGB 0136+391 and PKS 0735+178, during an epoch when the optical nucleus was in a faint state in both targets. In PKS 0735+178 we find a significant excess over a point source, which, if fitted by the de Vaucouleurs model, corresponds to a galaxy with I=18.64±0.11 and r_eff=1.8±0.4arcsec. Interpreting this galaxy as the host galaxy of PKS 0735+178 we derive z=0.45±0.06 using the host galaxy as a ``standard candle''. We also discuss the immediate optical environment of PKS 0735+178 and the identity of the MgII absorber at z=0.424. Despite of the optimally chosen epoch and deep imaging we find the surface brightness profile of RGB 0136+391 to be consistent with a point source. By determining a lower limit for the host galaxy brightness by simulations, we derive z>0.40 for this target.