  In a 15-GHz Ryle-Telescope observation of PC1643+4631 A & B, a pair of
quasars at redshifts z = 3.79 and 3.83 separated by 198 arcsec on the sky, we
find a decrement in the cosmic microwave background (CMB) of -380 +/- 64 uJy in
a 110 arcsec x 175 arcsec beam. Assuming this to be a Sunyaev-Zel'dovich effect
due to an intervening cluster, the minimum magnitude of the central temperature
decrement is 560 uK. A serendipitous ROSAT observation shows that there is no
X-ray-luminous cluster in the direction of the decrement at z < 1. The implied
gas mass is >~ 2 x 10^{14} solar masses (assuming a temperature of ~ 5 keV),
indicating a total mass of > 10^{15} solar masses. This result demonstrates the
existence of a massive system too distant to be detected by its emission, but
which can be found via its imprint on the CMB.
