  The emission line survey within the Calar Alto Deep Imaging Survey (CADIS)
detects galaxies with very low continuum brightness by using an imaging
Fabry-Perot interferometer. With spectroscopic follow-up observations of
MB>~-19 CADIS galaxies using FORS2 at the VLT and DOLORES at TNG we obtained
oxygen abundances of 5 galaxies at z~0.4 and 10 galaxies at z~0.64. Combining
these measurements with published oxygen abundances of galaxies with MB<~-19 we
find evidence that a metallicity-luminosity relation exists at medium redshift,
but it is displaced to lower abundances and higher luminosities compared to the
metallicity-luminosity relation in the local universe. Comparing the observed
metallicities and luminosities of galaxies at z<3 with Pegase2 chemical
evolution models we have found a favoured scenario in which the metallicity of
galaxies increases by a factor of ~2 between z~0.7 and today, and their
luminosity decreases by ~0.5-0.9mag.
