1. E. Witten, Some comments on string dynamics, hepth/9507121.
2. A. Strominger, Open p-branes, hepth/9512059.
3. O. Ganor and A. Hanany, Small instantons and tensionless strings, hepth/9602120.
4. Comments on string dynamics in six dimensions;Seiberg;Nucl. Phys. B,1996
5. M. Duff, H. Lu and C.N. Pope, Heterotic phase transitions and singularities of the gauge dyonic string, hepth/9603037.