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We present an extensive photometric and spectroscopic study of twohigh-redshift clusters of galaxies based on data obtained from the Keck 10mtelescopes and the Hubble Space Telescope. The clusters CL0023+0423 (z=0.84)and CL1604+4304 (z=0.90) are part of a multi-wavelength program to study ninecandidate clusters at z > 0.6 (Oke, Postman & Lubin 1998). Based on theseobservations, we study in detail both the field and cluster populations. Fromthe confirmed cluster members, we find that CL0023+0423 actually consists oftwo components separated by ~2900 km/s. A kinematic analysis indicates that thetwo components are a poor cluster with ~3 x 10^{14} solar masses and a lessmassive group with 10^{13} solar masses. CL1604+4304 is a centrallyconcentrated, rich cluster at z = 0.8967 with a velocity dispersion of 1226km/s and a mass of ~3 x 10^{15} solar masses. Approximately 57% and 50% of thegalaxies show high levels of star formation in CL0023+0423 and CL1604+4304,respectively. These numbers are significantly larger than those found inintermediate redshift clusters. We also observe many old, red galaxies. Foundmainly in CL1604+4304, they have spectra consistent with passive stellarevolution, typical of the early-type galaxies in low and intermediate-redshiftclusters. We have calculated their ages by comparing their spectral energydistributions to standard Bruzual & Charlot evolutionary models. We find thattheir colors are consistent with models having an exponentially decreasing starformation rate with a time constant of 0.6 Gyr. We observe a significantluminosity brightening in our brightest cluster galaxies. Compared to BCGs at z< 0.1, we find a luminosity increase of ~1 mag in the rest B-band and ~0.8 magin the rest V-band.Comment: 65 pages, 21 figures. (Two are large plate images available from the  lead author). Accepted for publication in the Astronomical Journa

A Study of Nine High-Redshift Clusters of Galaxies. II. Photometry, Spectra, and Ages of Clusters 0023+0423 and 1604+4304