A Unifying Topological Action for Heterotic and Type II Superstring Theories

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