1. M.B. Green, J.H. Schwarz and E. Witten, Superstring theory, Vol. 1&2, Cambridge University Press, 1988.
2. A. Dabholkar, G. Gibbons, J.A. Harvey and F. Ruiz Ruiz, Nucl. Phys. B340 33 (1990).
3. M. J. Duff, R. Khuri, and J.X. Lu, String Solitons, hep-th/9412184.
4. A. Strominger, Nucl. Phys. B274 253 (1986).
5. C. Callan, J.A. Harvey and A. Strominger, Nucl. Phys. B359 611 (1991)