  Explicit relations among moduli of the Heterotic and Type IIB string theories
in 8 dimensions are obtained. We identify the BPS states responsible for gauge
enhancements in the type IIB theory and their dual partners in the Heterotic
theory compactified with and without Wilson lines. The masses of BPS states in
Type IIB string theory compactified on the base space of a elliptically fibred
K3 are computed explicitly for the special cases in which the complex structure
of the fibre is constant, ie, for constant scalar fields backgrounds.
