  We discuss the equivalence between Type I, Type II and Heterotic N=2
superstring theories in four dimensions. We study the effective field theory of
Type I models obtained by orientifold reductions of Type IIB compactifications
on $K_3\times T^2$. We show that the perturbative prepotential is determined by
the one-loop corrections to the Planck mass and is associated to an index. As
is the case for threshold corrections to gauge couplings, this renormalization
is entirely due to N=2 BPS states that originate from D=6 massless string
modes. We apply our result to the so-called S-T-U model which admits
simultaneous Type II and Heterotic descriptions, and show that all three
prepotentials agree in the appropriate limits as expected from the superstring
triality conjecture.
