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String Dynamics at Strong Coupling

The dynamics of superstring, supergravity and M theories and their compactifications are probed by studying the various perturbation theories that emerge in the strong and weak coupling limits for various directions in coupling constant space. The results support the picture of an underlying non-perturbative theory that, when expanded perturbatively in different coupling constants, gives different perturbation theories, which can be perturbative superstring theories or superparticle theories. The $p$-brane spectrum is considered in detail and a criterion found to establish which $p$-branes govern the strong coupling dynamics. In many cases there are competing conjectures in the literature, and this analysis decides between them. In other cases, new results are found. The chiral six-dimensional theory resulting from compactifying the type IIB string on $K_3$ is studied in detail and it is found that certain strong coupling limits appear to give new theories, some of which hint at the possibility of a twelve-dimensional origin.