Galaxy population in distant galaxy clusters .1. Cl 0939+472 (z=0.41) and Cl 0016+161 (z=0.54)

*We present results of a study of the galaxy population of Cl 0939+472 (z = 0.41) and Cl 0016+161 (z = 0.54). We have used narrow-band filters (FWHM similar or equal to 90 - 200 Angstrom) and broad band B, R, I filters covering the range from 3800 Angstrom to 9200 Angstrom to obtain low resolution spectra for all galaxies brighter than R = 22.5 mag in a 5' x 5' (Cl 0939+472) and a 3.5' x 5' (Cl 0016+161) field. Template spectra for classical Hubble type and E + A galaxies were fitted to the low-resolution spectral energy distribution in order to determine the galaxies' redshift and the morphological type. We detected 160 cluster members in Cl 0939+472 and 100 in Cl 0016+161, with a success rate of about 80% in the determination of redshifts and corresponding classification of morphological types from spectral energy distributions. These results constitute a statistical improvement of at least a factor of 4 over the most complete study to date of these clusters. In particular, are provide a large sample of elliptical galaxies with secure membership, well suited for a study of evolutionary effects. The same E + A templates developed for the analysis of Cl 0939+472 were successful in recognizing almost all of the spectroscopically already known E + A galaxies in Cl 0016+161 and in identifying 10 new ones. Our results show that in this cluster, too, the fraction of E + A galaxies represents about 20% of the total galaxy population. This outlines the importance of taking into account galaxies with signs of recent star formation for a correct evaluation of the Butcher-Oemler effect in distant galaxy clusters.