String-dual F1-function in the three parameter model

The computation of the holomorphic F1-function describing one-loop gravitational couplings of vectormultiplets is shown to confirm the string-string duality of the proposed dual pair consisting of the heterotic string on K3 × T2 with gauge group U(1)4 and the type IIA string on the Calabi-Yau WP1,1,2,8,124(24).