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Poor clusters of galaxies are intermediate between the rare but well-studied rich clusters and the more common but less homogeneous loose groups. Recently, White et al. (1994) have suggested that the fraction of the mass in clusters which is baryonic is inconsistent with $\Omega=1$ and the primordial nucleosynthesis limits on $\Omega_b$. David et al. (1990) and Hughes (1989) have calculated the relative contributions of gas, galaxies, and dark matter in a few rich clusters (David et al. 1990, Hughes 1989), but so far little work has been done on poorer systems.
We use Rosat x-ray data for four poor clusters to calculate gas and total masses within $0.25 h^{-1}$ Mpc. We use new optical data to determine blue luminosities and velocity dispersions. We compute baryon fractions and the gas fractions for these systems and compare our results with those obtained for clusters.
