Holomorphic Couplings In Non-Perturbative String Compactifications

In this thesis we present an analysis of several aspects of four-dimensional, non-perturbative N=1 compactifications of string theory. Our focus is on the study of brane dynamics and their effective physics as encoded in the holomorphic couplings of the low-energy N=1 effective action, most prominently the superpotential W. The thesis is divided into three parts. In part one we derive the effective action of a spacetime-filling D5-brane in generic Type IIB Calabi-Yau orientifold compactifications. In the second part we invoke tools from string dualities, namely from F-theory, heterotic/F-theory duality and mirror symmetry, for a more elaborate study of the dynamics of (p,q) 7-branes and heterotic five-branes. In this context we demonstrate exact computations of the complete perturbative effective superpotential, both due to branes and background fluxes. Finally, in the third part we present a novel geometric description of five-branes in Type IIB and heterotic M-theory Calabi-Yau compactifications via a non-Calabi-Yau threefold \hat{Z}_3, that is canonically constructed from the original five-brane and Calabi-Yau threefold Z_3 via a blow-up. We exploit the use of the blow-up threefold \hat{Z}_3 as a tool to derive open-closed Picard-Fuchs differential equations, that govern the complete effective brane and flux superpotential. In addition, we present first evidence to interpret \hat{Z}_3 as a flux compactification dual to the original five-brane by defining an SU(3)-structure on \hat{Z}_3, that is generated dynamically by the five-brane backreaction