c=2 Rational Toroidal Conformal Field Theories via the Gauss Product
  We find a concise relation between the moduli $\tau, \rho$ of a rational Narain lattice $\Gamma(\tau,\rho)$ and the corresponding momentum lattices of left and right chiral algebras via the Gauss product. As a byproduct, we find an identity which counts the cardinality of a certain double coset space defined for isometries between the discriminant forms of rank two lattices. 
Shinobu Hosono, Bong H. Lian, Keiji Oguiso, Shing-Tung Yau