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Self-interacting fundamental strings and black holes

We study the size distribution of very massive close string states and the typical string configuration as one slowly increase the string coupling, both in the case of zero and of non-zero Neveu-Schwarz charges. The computations are performed rigorously in string theory, starting from quantities that are well-defined in the theory and therefore clarify previous works on the subject which were based on various approximation techniques. We find that, starting from a value of the coupling in agreement with the one predicted by the black hole correspondence principle, the string ensemble is dominated in any dimensions by compact states whose size is within the correspondent black hole horizon radius, which is of the order of the string scale at the black hole/string transition point.