One-loop and D-instanton corrections to the effective action of open string models

One-loop corrections to the gauge coupling and the gauge kinetic function in certain classes of four-dimensional D-brane models are computed. It is described how to determine D-instanton corrections to the superpotential and the gauge kinetic function in such models. The Affleck-Dine-Seiberg superpotential is rederived in string theory. The existence of a new class of multi-instantons, dubbed poly-instantons, is conjectured.