The Luminosity Function of MS2255.7+2039 at z=0.288

The luminosity function of MS2255.7+2039 at z=0.288 is determined down to a total magnitude of R ~ 24, corresponding to M_R ~ -17 (H_0 = 50 km/s/Mpc). The data are corrected for incompleteness and crowding using detailed simulations. We find that the luminosity function is steeper than a standard Schechter function at faint magnitudes, and shows an excess of galaxies below M_R ~ -19. After corrections for light loss and crowding, the data can be described by a sum of two Schechter functions, one with M_R = -22.8 and alpha = -1.0, and one steeper with M_R = -18.9 and alpha = -1.5, representing the dwarf population. A straight-line fit to the faint part yields a slope similar to the Schechter alpha = -1.5 of the dwarf population. The luminosity function of MS2255.7+2039 is compared to other clusters at lower redshifts, and does not show any significant difference. The redshift range for clusters in which increased number of dwarf galaxies have been found is therefore extended to higher redshifts.