  Since T-duality has been proved only perturbatively and most of the heterotic
states map into solitonic, non-perturbative, type II states, the 6-dimensional
string-string duality between the heterotic string and the type II string is
not sufficient to prove the S-duality of the former, in terms of the known
T-duality of the latter. We nevertheless show in detail that perturbative
T-duality, together with the heterotic-type II duality, does imply the
existence of heterotic H-monopoles, with the correct multiplicity and multiplet
structure. This construction is valid at a generic point in the moduli space of
heterotic toroidal compactifications.
