R-band imaging of the host galaxies of RGB BL Lacertae objects

We present optical R-band images of 100 BL Lacertae objects in the ROSAT-Green Bank (RGB) sample obtained at the Nordic Optical Telescope (NOT). For 62 objects we could not find any previously published images in the literature, of these 35 are new BL Lacs discovered during the RGB BL Lac survey. We were able to resolve the host galaxy in 62% of the objects. We analyze the host galaxies using a two-dimensional fitting procedure and tabulate the best-fit core magnitude and host galaxy parameters (magnitude, effective radius, ellipticity, position angle and shape parameter beta ) for each object. With two exceptions, all objects are better fit by a model representing an elliptical galaxy (beta = 0.25) than by a disk galaxy model (beta = 1.0). In the two exceptions the disk host fit is only marginally better than the elliptical host fit, however, and we do not find a single clear example of a disk-dominated host galaxy. The host galaxies have an average brightness M_R= -23.9 +/- 0.8 and average effective radius r_eff = 13.2 +/- 0.8 kpc. Their bulk properties are indistinguishable from normal elliptical and radio galaxies. The median beta is 0.18, well below the value 0.25 that is normally used to describe ellipticals. However, the difference may be caused by a selection effect due to the dependence of beta on host galaxy luminosity.