Redshift constraints for RGB 0136+391 and PKS 0735+178 from deep optical
  imaging
  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 +- 0.4 arcsec. 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.
