  We consider systems of magnetized D9 branes on orbifolds supporting N=1 gauge
theories. In such realizations, the matter multiplets arise from "twisted"
strings connecting different stacks of branes. The introduction of Euclidean
5-branes wrapped on the six-dimensional compact space leads to instanton
effects. We examine the interplay between the annuli diagrams with an E5
boundary and the holomorphicity properties of the effective low-energy
supergravity action which describes the system, including its instanton
corrections. Mostly based on arXiv:0709.0245.
