Detection of the host galaxy of S5 0716+714

We have acquired a deep i-band image of the BL Lacertae object S5 0716+714 while the target was in an low optical state. Due to the faintness of the nucleus, we were able to detect the underlying host galaxy. The host galaxy is measured to have an I-band magnitude of 17.5 ± 0.5 and an effective radius of (2.7 ± 0.8) arcsec. Using the host galaxy as a “standard candle”, we derive z = 0.31 ± 0.08 (1σ error) for the host galaxy of S5 0716+714. This redshift is consistent with the redshift z = 0.26 determined by spectroscopy for 3 galaxies close to S5 0716+714. The effective radius at z = 0.31 would be 12 ± 4 kpc, which is consistent with values obtained for BL Lac host galaxies. An optical spectrum acquired during the same epoch shows no identifiable spectral lines.