Nonperturbative corrections to 4D string theory effective actions from   SL(2,Z) duality and supersymmetry
  We find the D(-1) and D1-brane instanton contributions to the hypermultiplet moduli space of type IIB string compactifications on Calabi-Yau threefolds. These combine with known perturbative and worldsheet instanton corrections into a single modular invariant function that determines the hypermultiplet low-energy effective acction. 
Daniel Robles-Llana, Martin Rocek, Frank Saueressig, Ulrich Theis,   Stefan Vandoren