DOUBLY SUPERSYMMETRIC NULL STRINGS AND STRING TENSION GENERATION

We propose a twistor-like formulation of N = 1, D = 3, 4, 6 and 10 nul l superstrings. The model possesses N = 1 target space supersymmetry a nd n = D - 2 local worldsheet supersymmetry, the latter replaces the k appa-symmetry of the conventional approach to the strings. Adding a We ss-Zumino term to a null superstring action we observe a string tensio n generation mechanism [J.A. De AzcArraga, J.M. Izquierdo and P.K. Tow nsend, Phys. Rev. D 45 (1992) 3321; P.K. Townsend, Phys. Lett. B 277 ( 1992) 285; E.Bergshoeff, L.A.J. London and P.K. Townsend, Class. Quant um Grav. 9 (1992) 2545; and F. Delduc, A. Galperin, P. Howe and E. Sok atchev, Phys. Rev. D 47 (1992) 578]: the induced worldsheet metric bec omes nondegenerate and the resulting model turns out to be classically equivalent to the heterotic string.