  Recent progress in understanding modulus stabilization in string theory
relies on the existence of a non-renormalization theorem for the 4D
compactifications of Type IIB supergravity which preserve N=1 supersymmetry. We
provide a simple proof of this non-renormalization theorem for a broad class of
Type IIB vacua using the known symmetries of these compactifications, thereby
putting them on a similar footing as the better-known non-renormalization
theorems of heterotic vacua without fluxes. The explicit dependence of the
tree-level flux superpotential on the dilaton field makes the proof more subtle
than in the absence of fluxes.
