  Within the framework of phenomenological Lagrangians we construct the
effective action of QCD relevant for the study of semileptonic decays of
charmed mesons. Hence we evaluate the form factors of D -> P(0^-) l^+ nu_l at
leading order in the 1/N_C expansion and, by demanding their QCD-ruled
asymptotic behaviour, we constrain the couplings of the Lagrangian. The
features of the model-independent parameterization of form factors provided and
their relevance for the analysis of experimental data are pointed out.
