Abstract
The two-loop contribution to the type IIB low energy effective action term D 4 R 4 , predicted by SL ( 2 , Z ) 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 D 4 R 4 match exactly, thus providing an indirect check of SL ( 2 , Z ) duality. The two-loop heterotic low energy term D 2 F 4 is evaluated in string perturbation theory; its form is closely related to the D 4 R 4 term in type II, although its significance to duality is an open issue.
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