Gauge fields and fermions in tachyon effective field theories

In this paper we incorporate gauge fields into the tachyon field theory mod els for unstable D-branes in bosonic and in type-II string theories. The ch osen couplings yield massless gauge fields and an infinite set of equally s paced massive gauge fields on codimension one branes. A lack of a continuum spectrum is taken as evidence that the stable tachyon vacuum does not supp ort conventional gauge excitations. For the bosonic string model we find tw o possible solvable couplings, one closely related to Born-Infeld forms and the other allowing a detailed comparison to the open string modes on boson ic D-branes. We also show how to include fermions in the type-II model. The y localize correctly on stable codimension one branes resulting in bose-fer mi degeneracy at the massless level. Finally, we establish the solvability of a large class of models that include kinetic terms with more than two de rivatives.