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Gauged WZNW models and coset constructions in relation to string theories

We start by giving a brief introduction to string theory with emphasis on the example of the bosonic string. In order to fully appreciate string theory it is necessary to study the dynamics of the surface that the string traces out when moving in space-time. This is described by what is known as conformal field theory, which is the subject of chapter two. After this digression we return to string theory to give some insight into how the results of the articles contained in this thesis could provide with interesting ingredients for constructions of realistic string theories. This is also continued on to some extent in chapter seven and eight. The articles in this thesis are discussing mainly two subjects, gauged WZNW models and affine branching functions. The background for those issues is given in chapter seven and eight. In the articles contained in this thesis we use two major tools. First affine Lie algebras and its representation theory, and secondly BRST quantization. In chapter four and five we deal with affine Lie algebras, and in chapter six we try to illuminate some issues on BRST quantization.