We give a detailed discussion of the matching of the BPS states of het
erotic, type I and type I' theories in d = 9 for general backgrounds.
This allows us to explicitly identify these (composite) brane states i
n the type I' theory that lead to gauge symmetry enhancement at critic
al points in moduli space. An example is the enhancement of SO(16) x S
O(16) to E-8 x E-S. (C) 1998 Elsevier Science B.V.