Dualities, CPT Symmetry and Dimensional Reduction in String Theory

In this lecture we address the following issues in the context of string theories: i) The role played by S and T dualities in obtaining topological inflation in N=1 supergravity models, ii) A mechanism to generate the baryon asymmetry of the Universe based on the string interactions that violate CPT symmetry and iii) The quantum cosmology of the dimensionally reduced multidimensional Einstein-Yang-Mills system.