1. M. A. Virasoro:Phys. Rev. D.,1, 2933 (1970).
2. J. Scherk: Orsay preprint, Dec. 1970, to be published.
3. T. H. Burnett, D. J. Gross, A. Neveu, J. Scherk andJ. H. Schwarz:Phys. Lett.,32 B, 115 (1970).
4. E. Del Giudice andP. O. Vecchia: M.I.T. preprint, June 1970, to be published.
5. G. F. Chew andD. L. Snyder: L.R.L. preprint, January 1970.