Abstract :
We provide some additional evidence in favour of the strong — weak coupling duality between the SO(32) heterotic and type I superstring theories by comparing terms quartic in the gauge field strength in their low-energy effective actions. We argue that these terms should not receive higher-loop string corrections so that duality should relate the leading-order perturbative coefficients in the two theories. In particular, we demonstrate that the coefficient of the F4-term in the one-loop (torus) part of the SO(32) heterotic string action is exactly the same as the coefficient of the F4-term in the tree-level (disc) part of the type I action.
