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 spacetime supersymmetry. We then confirm the existence of IIA and
IIB supergravity solutions having the appropriate p-brane bound states
interpretation.Comment: 21 pages, Phyzzx, Minor corrections, Version that will appear in
Nucl. Phys.
