U-DUALITY MULTIPLETS AND NONPERTURBATIVE SUPERSTRING STATES

We employ an algebraic approach for unifying perturbative and nonpertu rbative superstring states on an equal footing, in the form of U-duali ty multiplets, at all excited string levels. In compactified type-IIA supertring theory we present evidence that the multiplet is labeled by two spaces, ''index'' space and ''base'' space, on which U acts witho ut mixing them. Both spaces are nonperturbative extensions of similar spaces that label perturbative T-duality multiplets. Base space consis ts of all the central charges of the 11D SUSY algebra, while index spa ce corresponds to representations of the maximal compact subgroup K su bset of U. This structure predicts the quantum numbers of the nonpertu rbative states. We also discuss whether and how U multiplets may coexi st with 11-dimensional multiplets that are associated with an addition al nonperturbative 11D structure that seems to be lurking behind in th e underlying theory.