Classical and Non-Perturbative Aspects of the Gauge Sector in Superstring Theory.

The gauge sector of superstring theory is examined with emphasis on properties related to instanton and monopole solutions. The sector differs from the usual Yang-Mills one in its coupling to a massless scalar, the dilaton, which plays the part of a dynamical coupling. It is shown that exact instanton and static monopole solutions do not exist, and the implications of this result are examined. The equations of motion for the dilaton are then solved in the presence of a Wen-Witten monopole, which is a topological magnetic charge arising as a consequence of Calabi-Yau compactification and the non-simply-connected nature of the six-dimensional manifold. Finite and minimum energy configurations of the system are identified and obtained. Corrections to the axionic (theta) term in the superstring Lagrangian are also determined in the presence of the monopole.