Optical and infrared photometry of late-type stars in the Pleiades.

New optical photometry has been obtained for 130 stars in the Pleiades fainter than than V approximately equal to 10 mag. New infrared photometry for 35 of those stars has also been obtained. Analysis of the optical data indicates that the photometric binary frequency for late-type stars in the cluster is 26 percent. That frequency is consistent with both the binary frequency for field G stars derived by Abt and Levy (1976) and the photometric binary frequency for high-mass stars in the Pleiades derived by Bettis (1975). The latter result indicates that binary frequency is nearly independent of the mass of the primary star over the range of M/solar mass between 0.4 and 4.0. The late G and K stars in the Pleiades fall systematically below a nominal main sequence derived from photometry of Hyades and Praesepe stars. Evidence is presented to ascribe that displacement to a metallicity difference between the Pleiades and Hyades stars, to slight IR excesses for the late-type Pleiades stars, and to an incorrect calibration of the cluster differential distance moduli. If the V approximately equal to 12-13 mag Pleiades stars are forced to lie on the zero-age main sequence, the Vandenberg et al. (1983) isochrones suggest a contraction age for low-mass stars in the Pleiades of slightly more than 100 million years. The difference between the nuclear age and the contraction age of the Pleiades would then be greater than a few times 10 million years, in agreement with the age spread of star formation estimated from rotational velocity data presented elsewhere.