Perturbative gravitational couplings and Siegel modular forms in   $D=4,N=2$ string models
  We consider four-parameter $D=4,N=2$ string models with Hodge numbers $(4,214- 12n)$ and $(4,148)$, and we express their perturbative Wilsonian gravitational coupling $F_1$ in terms of Siegel modular forms. 
G. L. Cardoso