Studies of rich clusters of galaxies - III. Photometry of the cluster A1930 and the m*-z relation.

Photometry is presented for objects in the central region of the cluster Al 930. We have no evidence of radial luminosity or colour segregation. The discontinuity in the integrated luminosity function occurs at a total absolute magnitude Mv* = - 2I'0. The extension of Bautz & Abell's data to higher redshift made by this cluster and A141 3 confirms that Mv* is a better distance indicator than the apparent magnitude of the brightest cluster member. The low dispersion of Mv* is used directly to obtain a value for the Hubble constant at the Virgo cluster of H0(VC) = 62 + 19 km 1 Mpc-1.