The low-energy S-wave component of the decay $D^+ to K^- pi^+ pi^+$ is studied by means of a chiral SU(3)XSU(3) effective theory. As far as the primary vertex is concerned, we allow for the possibility of either direct production of three pseudoscalar mesons or a meson and a scalar resonance. Special attention is paid to final state interactions associated with elastic meson-meson scattering. The corresponding two-body amplitude is unitarized by ressumming s-channel diagrams and can be expressed in terms of the usal phase shifts $delta$. This procedure preserves the chiral properties of the amplitude at low-energies. Final state interactions also involve another phase $omega$, which describes intermediate two-meson propagation and is theoretically unambiguous. This phase is absent in the K-matrix approximation. Partial contributions to the decay amplitude involve a real term, another one with phase $delta$ and several others with phases $delta+omega$. Our main result is a simple and almost model independent chiral generalization of the usual Breit-Wigner expression, suited to be used in analyses of production data involving scalar resonances.
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