We report the detection of a massive high-redshift cluster of galaxies near
the quadruple quasar RX J0911+05, using the LRIS instrument on the Keck-II
telescope. The cluster is found to have a mean redshift of =0.7689+/-0.002
and a velocity dispersion of sigma=836{+180-200} km/s, based on redshift
measurements for 24 member galaxies. This massive high-redshift cluster is the
origin of the unusually large external shear required by lensing models of the
quadruple quasar system. We predict the expected time delay depending on the
exact contribution of the cluster. A measurement of the time delay and further
deep lensing and X-ray observations will unravel useful properties of this
serendipitously discovered high-redshift cluster, and may put interesting
cosmological constraints on H0.Comment: Submitted to ApJL, 7 pages, 5 figure