ON SO(32) HETEROTIC - TYPE-I SUPERSTRING DUALITY IN 10 DIMENSIONS

We provide some additional evidence in favour of the strong - weak cou pling duality between the SO(32) heterotic and type I superstring theo ries by comparing terms quartic in the gauge field strength in their l ow-energy effective actions. We argue that these terms should not rece ive higher-loop string corrections so that duality should relate the l eading-order perturbative coefficients in the two theories. In particu lar, we demonstrate that the coefficient of the F-4-term in the one-lo op (torus) part of the SO(32) heterotic string action is exactly the s ame as the coefficient of the F-4-term in the tree-level (disc) part o f the type I action.