Counting Curves with Modular Forms

We consider the type IIA string compactified on the Calabi-Yau space given by a degree 12 hypersurface in the weighted projective space ${\bf P}^4_{(1, 1, 2,2, 6)}$. We express the prepotential of the low-energy effective supergravity theory in terms of a set of functions that transform covariantly under $PSL(2, \mathbb{Z})$ modular transformations. These functions are then determined by monodromy properties, by singularities at the massless monopole point of the moduli space, and by $S \leftrightarrow T$ exchange symmetry.