  U duality transformations must act on a basis of states that form complete
multiplets of the U group, at any coupling, even though the states may not be
mass degenerate, as for a broken symmetry. Similarly, if superstring theory is
related to a non-perturbative 11D M-theory, then an 11D supermultiplet
structure is expected, even though the multiplet may contain states of
different masses. We analyse the consistency between these two multiplet
schemes at the higher excited string levels for various compactifications of
the type IIA superstring. While we find complete consistency for a number of
compactifications, there remain some unanswered questions in others. The
relation to D-branes also needs further clarification.
