Prepotentials from Symmetric Products

We investigate the prepotential that describes certain F^4 couplings in eight dimensional string compactifications, and show how they can be computed from the solutions of inhomogenous differential equations. These appear to have the form of the Picard-Fuchs equations of a fibration of Sym^2(K3) over P^1. Our findings give support to the conjecture that the relevant geometry which underlies these couplings is given by a five-fold.