  Using the topological techniques developed in an earlier paper with Vafa, a
field theory action is constructed for any open string with critical N=2
worldsheet superconformal invariance. Instead of the Chern-Simons-like action
found by Witten, this action resembles that of a Wess-Zumino-Witten model. For
the N=2 string which describes (2,2) self-dual Yang-Mills, the string field
generalizes the scalar field of Yang.
  As was shown in recent papers, an N=2 string can also be used to describe the
Green-Schwarz superstring in a Calabi-Yau background. In this case, one needs
three types of string fields which generalize the real superfield of the
super-Yang-Mills prepotential, and the chiral and anti-chiral superfields of
the Calabi-Yau scalar multiplet. The resulting field theory action for the open
superstring in a Calabi-Yau background has the advantages over the standard RNS
action that it is manifestly SO(3,1) super-Poincar\'e invariant and does not
require contact terms to remove tree-level divergences.
