  A search for fossil groups in the Sloan Digital Sky Survey was performed
using virtual observatory tools. A cross-match of the positions of all SDSS
Luminous Red Galaxies (with r < 19 and measured spectroscopic redshifts) with
sources in the ROSAT All-Sky Survey catalog resulted in a list of elliptical
galaxies with extended X-ray emission (with a galaxy/ROSAT-source distance of
less than 0.5 arcmin in all cases). A search for neighbors of the selected
elliptical galaxies within a radius of $0.5 h_{70}^{-1}$ Mpc was conducted
taking into account the r-band magnitudes and spectroscopic or photometric
redshifts of all objects within this area, leading to a sample of 34 candidate
fossil groups. Considering this sample, the estimated space density of fossil
systems is $n =(1.0 \pm 0.6) \times 10^{-6}$ $h_{50}^3$ Mpc$^{-3}$.
