Stringy Instantons in IIB Brane Systems
  In this note, we study D-brane instantons which intersect N=1 supersymmetric configurations of space-filling D-branes in general Calabi-Yau compactifications of type IIB string theory. Our focus is on rigid ``stringy instantons'' -- those which cannot be interpreted as Yang-Mills instantons in a non-Abelian gauge group on any stack of space-filling D-branes. We show that their contributions to the space-time superpotential can be determined by a topological B-model computation of the tree-level superpotential for a related auxiliary brane system. This computation is very tractable, as it is governed by an A-infinity structure. We summarize the stringy instanton contribution to the space-time superpotential in a simple formula. 
Shamit Kachru and Dusan Simic