  A decrement in the Cosmic Microwave Background (CMB) has been observed by the
Ryle Telescope towards a pair of, possibly lensed, quasars (PC1643+4631 A&B).
Assuming that the decrement is due to the Sunyaev-Zel'dovich (S-Z) effect, this
is indicative of a very rich intervening cluster, although no X-ray emission
has yet been observed in that direction. In order to investigate these
problems, we present a new model for the formation of distant spherically
symmetric clusters in an expanding Universe. Computation of photon paths allows
us to evaluate the gravitational effects on CMB photons passing through the
evolving mass (i.e. Rees Sciama effect). The lensing properties of the cluster
are also considered so that the model can be applied to the PC1643+4631 case to
retrieve both the S-Z flux and the separation of the quasar pair. We find that
the Rees Sciama effect might contribute significantly to the overall observed
CMB decrement.
