BPS States and Non-Perturbative Aspects of String Theory

In a first chapter we briefly review some features of ten dimensional string dualities, BPS bounds and D-brane physics which will set the background and notations for the later discussions. In chapter 2 we study the spectrum of D-string bound states in the context of type I/heterotic and IIB self- dualities. Chapter 3 is devoted to the computation of "BPS saturated" threshold corrections in type IIB ( 4,0) string vacua (four refers to the number of four-dimensional supersymmetries). Finally in chapter 4 the SYM/SUGRA correspondence is discussed for a class of D-brane configurations describing spin effects and nontrivial fluxes/background data. Finally some conclusions and discussion are included