EVIDENCE FOR HETEROTIC - TYPE-I STRING DUALITY

A study is made of the implications of heterotic string T-duality and extended gauge symmetry for the conjectured equivalence of heterotic a nd Type I superstrings. While at first sight heterotic string world-sh eet dynamics appears to conflict with Type I perturbation theory, a cl oser look shows that Type I perturbation theory ''miraculously'' break s down, in some cases via novel mechanisms, whenever the heterotic str ing has massless particles not present in Type I perturbation theory. This strongly suggests that the two theories actually are equivalent. As further evidence in the same direction, we show that the Dirichlet one-brane of Type I string theory has the same world-sheet structure a s the heterotic string.