TWISTOR FORMULATION OF THE HETEROTIC D = 10 SUPERSTRING WITH MANIFEST (8,0) WORLD-SHEET SUPERSYMMETRY

We propose a new formulation of the heterotic D = 10 Green-Schwarz superstring whose world sheet is a superspace with two even and eight odd coordinates. The action is manifestly invariant under both target-space supersymmetry and a world-sheet reparametrization supergroup. It contains only a finite set of auxiliary fields. The key ingredients are the commuting spinor (twistor) variables, which naturally arise as world-sheet superpartners of the target-space Grassmann coordinates. These spinors parametrize the sphere S8 regarded as a coset space of the D = 10 Lorentz group. The sphere is associated with the lightlike vector of one of the string Virasoro constraints. The origin of the on-shell D = 10 fermionic kappa symmetry of the standard Green-Schwarz formulation is explained. An essential and unusual feature is the appearance of the string tension only on shell as an integration constant.