On Modular Invariant Partition Functions for Tensor Products of   Conformal Field Theories
  We give two results concerning the construction of modular invariant partition functions for conformal field theories constructed by tensoring together other conformal field theories. First we show how the possible modular invariants for the tensor product theory are constrained if the allowed modular invariants of the individual conformal field theory factors have been classified. We illustrate the use of these constraints for theories of the type $SU(2)_{K_A}\otimes SU(2)_{K_B}$, finding all consistent theories for $K_A,K_B$ odd. Second we show how known diagonal modular invariants can be used to construct some inherently asymmetric ones where the holomorphic and anti-holomorphic theories do not share the same chiral algebra. Some explicit examples are given. 
Gerald B. Cleaver and David C. Lewellen