Observations of the host galaxies of the BL Lacertae objects H 0414+009 and OJ 287 with FORS1 at VLT-UT1

We present imaging of the host galaxies of H 0414+009 and OJ 287 taken duri ng the commissioning phases of FORS1 at the VLT-UT1. The host galaxy of H 0 414+009 is well resolved in R and I, and marginally resolved in B. Based on our 2-dimensional modelling, it is a large and very luminous elliptical ga laxy (M-R=-24.6, r(e)=35 kpc) with colours commonly found in elliptical gal axies (B-R similar to 1.45 and R-I similar to 0.45). The results are consis tent with previous findings. The host galaxy of OJ 287 is marginally resolved. A de Vaucouleurs profile is slightly preferred over a disk-type profile. If true, the host galaxy is luminous (M-R=-23.4), compact (r(e)=4.4 kpc) and has a small offset to the south with respect to the active nucleus. This is consistent with the resu lts obtained by Sillanpaa et al. (1999), but different to the findings by W urtz et al. (1996) and Yanny et al. (1997). The analysis of the marginally resolved host galaxies was complicated by a point spread function (PSF) varying across the field. Consequently, better results were obtained when using a PSF which varies with field position as compared to a constant PSE This is possible as long as a sufficient number of stars is present on the images. Masking the central regions mostly affec ted by the PSF errors may also improve the results. Based on our experience we describe precaution which has to be taken for FORS1 observations and su bsequent analysis of marginally resolved targets such as AGN host galaxies or planetary nebulae around young stellar objects.