A UNIFYING TOPOLOGICAL ACTION FOR HETEROTIC AND TYPE-II SUPERSTRING THEORIES

The heterotic and type II superstring actions are identified in differ ent anomaly-free decompositions of a single topological sigma-model ac tion depending on bosonic and fermionic coordinates, X(mu) and rho(A) respectively, and of their topological ghosts. This model results from gauge-fixing the topological gauge symmetry delta X(mu) = epsilon(mu) (z, (z) over bar) (mu = 1, 2,..., 10) and delta mu(alpha) = epsilon(a lpha) (z, (z) over bar) (alpha = 1, 2,..., 16). From another viewpoint the heterotic and type H superstring actions emerge as two different gauge-fixings of the same closed two-form. Comments are also made conc erning the possibility of relating rho(alpha) to a Majorana-Weyl space -time spinor superpartner of X(mu).