  For a detailed comparison of the appearance of cluster of galaxies in X-rays
and in the optical, we have compiled a comprehensive database of X-ray and
optical properties of a sample of clusters based on the largest available X-ray
and optical surveys: the ROSAT All Sky Survey (RASS) and the Sloan Digital Sky
Survey (SDSS). The RASS-SDSS sample comprises 114 X-ray selected clusters. For
each system we have uniformly determined the X-ray and optical properties . For
a subsample of 53 clusters we have also compiled the temperature and the iron
abundance from the literature. The total optical luminosity can be determined
with a typical uncertainty of 20% with a result independent of the choice of
local or global background subtraction. We searched for parameters which
provide the best correlation between the X-ray luminosity and the optical
properties and found that the z band luminosity determined within a cluster
aperture of 0.5 Mpc h_{70}^{-1} provides the best correlation with a scatter of
about 60%. The resulting correlation of L_X and L_{op} in the z and i bands
shows a logarithmic slope of 0.38, a value not consistent with the assumption
of a constant M/L. Consistency is found, however, for an increasing M/L with
luminosity. This is confirmed by our analysis of the cluster mass to light
ratio for a subsample of 53 clusters. We found a significant dependence of M/L
on the cluster mass with a logarithmic slope ranging from 0.27 in the i and r
bands to 0.22 in the z band.
