Stringy instantons and quiver gauge theories

We explore contributions to the 4D effective superpotential which arise form Euclidean D3 branes (instantons) that intersect space-filling D-branes. These effects can perturb the effective field theory on the space-filling branes by nontrivial operators composed of charge matter fields, changing the vacuum structure in a qualitative was in some examples. Our considerations are exemplified throughout by a careful study of a fractional brane configuration on a del Pezzo surface.