Two-Loop Superstrings VI: Non-Renormalization Theorems and the 4-Point   Function
  The N-point amplitudes for the Type II and Heterotic superstrings at two-loop order and for $N \leq 4$ massless NS bosons are evaluated explicitly from first principles, using the method of projection onto super period matrices introduced and developed in the first five papers of this series. The gauge-dependent corrections to the vertex operators, identified in paper V, are carefully taken into account, and the crucial counterterms which are Dolbeault exact in one insertion point and de Rham closed in the remaining points are constructed explicitly. This procedure maintains gauge slice independence at every stage of the evaluation.   Analysis of the resulting amplitudes demonstrates, from first principles, that for $N\leq 3$, no two-loop corrections occur, while for N=4, no two-loop corrections to the low energy effective action occur for $R^4$ terms in the Type II superstrings, and for $F^4$, $F^2F^2$, $F^2R^2$, and $R^4$ terms in the Heterotic strings. 
Eric D'Hoker and D.H. Phong