Two-loop superstrings and S-duality Original Research Article

The two-loop contribution to the type IIB low energy effective action term image, predicted by image duality, is compared with that of the two-loop 4-point function derived recently in superstring perturbation theory through the method of projection onto super period matrices. For this, the precise overall normalization of the 4-point function is determined through factorization. The resulting contributions to image match exactly, thus providing an indirect check of image duality. The two-loop heterotic low energy term image is evaluated in string perturbation theory; its form is closely related to the image term in type II, although its significance to duality is an open issue.