1. C. Vafa, Evidence for F-theory, hepth/9602022.
2. D. Morrison and C. Vafa, Compactifications of F-theory on Calabi-Yau threefolds. (I), (II), hepth/9602114, 9603161.
3. E. Witten, Some comments on string dynamics, hepth/9507121.
4. A. Strominger, Open p-branes, hepth/9512059.
5. O. Ganor and A. Hanany, Small instantons and tensionless strings, hepth/9602120.