We show that the M-theory/IIA and IIA/IIB superstring dualities together with the diffeomorphism invariance of the underlying theories require the presence of certain p -brane bound states in IIA and IIB superstring theories preserving 1 2 of the space-time supersymmetry. We then confirm the existence of IIA and IIB supergravity solutions having the appropriate p -brane bound states interpretation.
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