CONSISTENCY BETWEEN 11D AND U-DUALITY

U duality transformations must act on a basis of states that form comp lete multiplets of the U group, at any coupling, even though the state s 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 multiple t may contain states of different masses. We analyse the consistency b etween these two multiplet schemes at the higher excited string levels for various compactifications of the type IIA superstring. While we f ind complete consistency for a number of compactifications, there rema in some unanswered questions in others. The relation to D-branes also needs further clarification.