Aspects of plane waves and Taub-NUT as exact string theory solutions

This thesis is a study of some aspects of string theory solutions that are exact in the inverse string tension ɑ', and thus are valid beyond the low-energy limit. I investigate D-brane interactions in the maximally supersymmetric plane wave solution of type IIb string theory, and study the fate of the stringy halo surrounding D-branes. I find that the halo is like in flat space for Lorentzian D-branes, while it receives a non-trivial modification for Euclidean D-branes. I also comment on the connection between Hagedorn temperature and T-duality, which motivates a more general study of T-duality in null directions. I consider such transformations in a spinning D-brane solution of supergravity, and find that divergences in the field components associated with null T-dualities are invisible to string and brane probes. I also observe that there are closed timelike curves in all the T-dual solutions, but that none of them are geodesies. The second half of the theses is an investigation of the fate of closed tirnelike curves and of cosmological singularities in an exact stringy Taub-NUT solution of heterotic string theory, and in a rotating generalisation of it. I compute the exact spacetime fields, using a description in terms of a gauged Wess-Zumino-Novikov-Witten model and find that the ɑ' corrections are mild. The key features of the Taub-NUT geometry persist, together with the emergence of a new region of space with Euclidean signature. Closed timelike curves are still present, which is inter-preted as a sign that they might be a natural ingredient in string theory, for instance in pre-Big-Bang cosmological scenarios