Interacting field theory of open superstrings

A proposal for a gauge invariant nonlinear classical field theory of open superstrings is explored. The gauge fixing that gives rise to the standard structure in the Ramond sector is described, answering questions that have been raised in recent literature. The model is shown to possess space-time supersymmetry as well as gauge invariance. The proof of space-time supersymmetry uses the covariant form of the fermion emission vertex and the associated picture changing operation, in accord with suggestions by several authors. Also discussed are the canonical formulation of the theory, and some issues connected with the extension of the work to closed strings.