Bound states of type I D-strings

We study the infrared limit of the O(N) gauge theory that describes the low energy modes of a system of N type I D-strings and provide some support to the conjecture that, in this limit, the theory flows to an orbifold conformal theory. We compute the elliptic genus of the orbifold theory and argue that its longest string sector describes the bound states of D-strings. We show that, as a result, the masses and multiplicities of the bound states are in agreement with the predictions of heterotic-type I duality in nine dimensions, for all the BPS charges in the lattice Gamma((1,17)) (C) 1998 Elsevier Science B.V.