U-duality and Intersecting D-branes

Abstract

Spectrum of elementary string states in type II string theory compactified on a torus contains short multiplets which are invariant under only one quarter of the space-time supersymmetry generators. $U$-duality transformation converts these states into bound states of Dirichlet branes which wrap around intersecting cycles of the internal torus. We study a class of these bound states that are dual to the elementary string states at the first excited level, and argue that the degeneracy of these bound states is in agreement with the $U$-duality prediction.