Key points are not available for this paper at this time.
A simple proof of the nonrenormalization theorems in string theory is given. We prove (with the assumption that space-time supersymmetry is nonanomalous) that the space-time superpotential is not renormalized by string loops. As corollaries of this result we argue that in string perturbation theory, Calabi-Yau vacua (or any other classical solution with space-time supersymmetry) are solutions of the quantum equations of motion with zero cosmological constant, massless particles remain massless, and Yukawa couplings are not renormalized, to all orders.
Dine et al. (Mon,) studied this question.