Vanishing Theorems for the Self-Dual N=2 String
  It is proven that up to possible surface terms, the only non-vanishing momentum-dependent amplitudes for the self-dual N=2 string in $R^{2,2}$ are the tree-level two and three-point functions, and the only non-vanishing momentum-independent amplitudes are the one-loop partition function and the tree-level two and four-point functions. The calculations are performed using the topological prescription developed in an earlier paper with Vafa. As in supersymmetric non-renormalization theorems, the vanishing proof is based on a relationship between the zero-momentum dilaton and axion. 
Nathan Berkovits