Gauge-covariant S-matrices for field theory and strings
  S-matrices can be written Lorentz covariantly in terms of free field strengths for vector states, allowing arbitrary gauge choices. In string theory the vertex operators can be chosen so this gauge invariance is automatic. As examples we give four-vector (super)string tree amplitudes in this form, and find the field theory actions that give the first three orders in the slope. 
Haidong Feng, Warren Siegel