Wide-field optical and near--IR (JHK) imaging is presented for two rich
galaxy clusters: Abell~370 at z=0.374 and Abell~851 (Cl0939+47) at z=0.407.
Galaxy catalogs selected from the near--IR images are 90\% complete to
approximately 1.5 mag below K∗ resulting in samples with ∼100
probable member galaxies per cluster in the central ∼2 Mpc. Comparison
with HST WFPC images yields subsamples of ∼70 galaxies in each cluster
with morphological types. Analysis of the complete samples and the HST
subsamples shows that the z∼0.4 E/S0s are bluer than those in the Bower
et al.\ (1992) Coma sample in the optical−K color by 0.13~mag for Abell~370
and by 0.18~mag for Abell~851. If real, the bluing of the E/S0 populations at
moderate redshift is consistent with that calculated from the Bruzual and
Charlot (1993) models of passive elliptical galaxy evolution. In both clusters
the intrinsic scatter of the known E/S0s about their optical−K color--mag
relation is small (∼0.06 mag) and not significantly different from that
of Coma E/S0s as given by Bower et al.\ (1992), indicating that the galaxies
within each cluster formed at the same time at an early epoch.Comment: uuencoded gzipped tar file containing latex files of manuscript (42
pages) plus tables (9 pages); figures available by anonymous ftp at
ftp://ipac.caltech.edu//pub/pickup/sed ; accepted for publication in the Ap