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Color-magnitude relations have been derived in the clusters RX J0849+4452 andRX J0848+4453 at z~1.26. The color-magnitude relation was determined fromAdvanced Camera for Surveys imaging in the WFC F775W (i_775) and F850LP (z_850)filters combined with ground-based spectroscopy. Early-type cluster candidateshave been identified according to the Postman et al. morphologicalclassification. In both clusters the bright red early-type population defines atight color-magnitude relation very similar in color, although the two clusterspresent different X-ray luminosities and shapes, with RX J0849+4452 being threetimes more X-ray luminous and more compact, and having a temperature two timeshigher. The elliptical galaxy color-magnitude relations (CMR) in RX J0849+4452and RX J0848+4453 show an intrinsic (i_775-z_850) color scatter of 0.026 +/-0.012 mag and 0.024 +/- 0.015 mag, respectively, within 2 arcminutes (~1Mpc atz=1.26) from the cluster X-ray emission centers. Simple modeling of thescatters using stellar population models from Bruzual and Charlot, gives a meanluminosity-weighted age t > 2.5 Gyr (z_f > 2.75). S0 galaxies follow theelliptical CMR; they show larger scatters about the CMR.  The intrinsic scatter decreases and the CMR slopes are steeper at smallerradii, within both clusters. We conclude that old stellar populations incluster elliptical galaxies are already in place at z=1.26, both in the moreevolved cluster RX J0849+4452, and in its less evolved companion RX J0848+4453.Even at a lookback time of 9 Gyr, in the early merging and buildup of massiveclusters, the bulk of the stellar content of the bright elliptical galaxypopulation was in place - apparently formed some 2.5~Gyr earlier at z~3Comment: 22 pages, 8 figures; ApJ, in pres

Evolution of the Color‐Magnitude Relation in High‐Redshift Clusters: Early‐Type Galaxies in the Lynx Supercluster atz∼ 1.26