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   71162
   The interferometric observation with the 15 GHz Ryle Telescope of the
   quasar pair PC 1643+4631 A, B (z = 3.79, 3.83, image separation 198'')
   shows a decrement in the cosmic microwave background (CMB) in the
   direction, which is due to a cluster with mass >~ 10^15 M_SUN at a
   redshift z >~1 and might also be responsible for the double image of a
   single quasar. The ROSAT PSPC observation at the position of the
   decrement shows that there is no evidence for emission by hot gas which
   could give rise to the Sunyaev-Zel'dovich effect. We propose deep field
   survey at the position of the decrement with ASCA to detect evidence
   for the distant cluster.
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