2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/56339We review quantitative tests on the duality between the heterotic string on T^2 and F-theory on K3. On the heterotic side, certain threshold corrections to the effective action can be exactly computed at one-loop order, and the issue is to reproduce these from geometric quantities pertaining to the K3 surface. In doing so we learn about certain non-perturbative interactions of 7-branes.latex, 10 figs, contribution to the proceedings of Cargese 1999High Energy Physics - Theorytext