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We study the mass-to-light ratio of galaxy systems from poor groups to richclusters, and present for the first time a large database for usefulcomparisons with theoretical predictions. We extend a previous work, where B_jband luminosities and optical virial masses were analyzed for a sample of 89clusters. Here we also consider a sample of 52 more clusters, 36 poor clusters,7 rich groups, and two catalogs, of about 500 groups each, recently identifiedin the Nearby Optical Galaxy sample by using two different algorithms. Weobtain the blue luminosity and virial mass for all systems considered. Wedevote a large effort to establishing the homogeneity of the resulting values,as well as to considering comparable physical regions, i.e. those includedwithin the virial radius. By analyzing a fiducial, combined sample of 294systems we find that the mass increases faster than the luminosity: the linearfit gives M\propto L_B^{1.34 \pm 0.03}, with a tendency for a steeper increasein the low--mass range. In agreement with the previous work, our presentresults are superior owing to the much higher statistical significance and thewider dynamical range covered (about 10^{12}-10^{15} M_solar). We present acomparison between our results and the theoretical predictions on the relationbetween M/L_B and halo mass, obtained by combining cosmological numericalsimulations and semianalytic modeling of galaxy formation.Comment: 25 pages, 12 eps figures, accepted for publication in Ap

Observational Mass‐to‐Light Ratio of Galaxy Systems from Poor Groups to Rich Clusters