We present the Fundamental Plane (FP) for 38 early-type galaxies in the two rich galaxy clusters RXJ0152.7-1357 (z=0.83) and RXJ1226.9+3332 (z=0.89), reaching a limiting magnitude of M_B =-19.8 mag in the rest frame of the clusters. While the zero point offset of the FP for these high redshift clusters relative to our low redshift sample is consistent with passive evolution with a formation redshift of z_form ~ 3.2, the FP for the high redshift clusters is not only shifted as expected for a mass-independent z_form, but rotated relative to the low redshift sample. Expressed as a relation between the galaxy masses and the mass-to-light ratios the FP is significantly steeper for the high redshift clusters than found at low redshift. We interpret this as a mass dependency of the star formation history, as has been suggested by other recent studies. The low mass galaxies (10^10.3 M_sun) have experienced star formation as recently as z ~ 1.35 (1.5 Gyr prior to their look back time), while galaxies with masses larger than 10^11.3 M_sun had their last major star formation episode at z > 4.5.
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