  We compute the gravitational coupling $F_1$ for IIA string theory on $K3
\times T^2$ and use string-string duality to deduce the corresponding term for
heterotic string on $T^6$. The latter is an infinite sum of gravitational
instanton effects which we associate with the effects of Euclidean fivebranes
wrapped on $T^6$. These fivebranes are the neutral fivebranes or zero size
instantons of heterotic string theory.
