Lüst, D., & Theisen, S. (1989). Superstring partition functions and the characters of exceptional groups. Physics Letters B, 227(3-4), 367-372. doi:10.1016/0370-2693(89)90945-3.
Abstract: We study the relation between the one-loop partition function of supersymmetric four-dimensional heterotic string theories and the level-one characters of exceptional groups. This is used to derive identities among θ-functions which show that the one-loop partition functions vanish identically. It is also shown that the exceptional groups provide the KaImage -Moody characters of the underlying conformal field theories.