Dual string with U(1) colour symmetry

The string model with a U(1) colour symmetry is analyzed in detail. It is shown that the critical dimension of this model is D=2. There are no tachyons in the U(1) string. Moreover the massless ground state is the only physical particle of the spectrum. All other states are decoupled because of the gauge conditions. The n-point amplitudes of the U(1) string are derived and shown to be equivalent to the tree diagrams of a conventional non-polynomial field theory in two dimensions, namely the non-linear sigma model where the pion particle is identified with the massless ground state of the string with an isotopic Chan-Paton symmetry. (Auth.)