Theisen, Stefan Quantum Gravity & Unified Theories, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
Lauer, J., Lüst, D., & Theisen, S. (1988). Supersymmetric string theories, superconformal algebras and exceptional groups. Nuclear Physics B, 309(4), 771-790. doi:10.1016/0550-3213(88)90340-9.