1. S. Kachru, J. Kumar and E. Silverstein, Vacuum energy cancellation in a non-supersymmetric string, hep-th/9807076.
2. Construction of fermionic string models in four dimensions
3. Four-dimensional superstrings
4. J. Harvey, Stringy duality and non-supersymmetric strings, hep-th/9807213.
5. S. Kachru and E. Silverstein, Self-dual non-supersymmetric Type II string compactifications, hep-th/9808056.