The connection between IIA superstring theory compactified on a circle of r
adius R and IIB theory compactified on a circle of radius 1/R is reexamined
from the perspective of N = 2, D = 9 space-time supersymmetry. We argue th
at the consistency of IIA/B duality requires the BPS states corresponding t
o momentum and winding of either of the type-II superstrings to transform a
s inequivalent supermultiplets. We show that this is indeed the case far an
y finite compactification radius, thus providing a nontrivial confirmation
of IIA/B duality. From the point of view of N = 2, D = 9 supergravity, one
is naturally led to an SL(2,Z) invariant field theory that encompasses both
the M-theory torus and the Kaluza-Klein states of the IIB theory. (C) 1999
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