Direct observation of the fourth star in the zeta Cancri system

Direct imaging of the zeta Cnc system has resolved the fourth star in the s ystem, which is in orbit around zeta Cnc C. The presence of the fourth star has been inferred for many years from irregularities in the motion of star C and recently from C's spectroscopic orbit. However, its mass is close to that of C, making its nondetection puzzling. Observing at wavelengths of 1 .2, 1.7, and 2.2 mu m with the adaptive optics system of the Canada-France- Hawaii Telescope, we have obtained images which very clearly reveal star D and show it to have the color of an M2 star. Its brightness is consonant wi th its being two M stars, which are not resolved in our observations but ar e likely to be in a short-period orbit, thereby accounting for the large ma ss and the difficulty of detection at optical wavelengths, where the magnit ude difference is much larger. The positions and colors of all four stars i n the system are reported and are consistent with the most recent astrometr ic observations.