  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. 
Invisible Clusters and CMB Decrements