  We give a simple derivation of the conformal blocks of the singleton sector
of compactifications of IIB string theory on spacetimes of the form X5 x Y5
with Y5 compact, while X5 has as conformal boundary an arbitrary 4-manifold M4.
We retain the second-derivative terms in the action for the B,C fields and thus
the analysis is not purely topological. The unit-normalized conformal blocks
agree exactly with the quantum partition function of the U(1) gauge theory on
the conformal boundary. We reproduce the action of the magnetic translation
group and the SL(2,Z) S-duality group obtained from the purely topological
analysis of Witten. An interesting subtlety in the normalization of the IIB
Chern-Simons phase is noted.
