Matching the BPS spectra of heterotic–type I–type I′ strings

We give a detailed discussion of the matching of the BPS states of heterotic, type I and type I′ theories in d=9 for general backgrounds. This allows us to explicitly identify these (composite) brane states in the type I′ theory that lead to gauge symmetry enhancement at critical points in moduli space. An example is the enhancement of SO(16)×SO(16) to E8×E8.