BRST properties of spin fields

For the closed superstring, spin fields and bi-spinor states are defined directly in four spacetime dimensions. Explicit operator product expansions are given, including those for the internal superconformal field theory, which are consistent with locality and BRST invariance for the string vertices. The most general BRST picture changing for these fields is computed. A covariant notation for the spin decomposition of these states is developed in which non-vanishing polarizations are selected automatically. The kinematics of the three-gluon dual model amplitude in both the Neveu-Schwarz and Ramond sectors in the Lorentz gauges is calculated and contrasted. Modular invariance and enhanced gauge symmetry of four-dimensional models incorporating these states is described.