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The vector form factor f_+(t) of the semileptonic decay D -> K l nu, measuredrecently with a high accuracy, can be used to determine the strong couplingconstant g_{D_s^* D K}. The latter is related to the normalised coupling\hat{g} releveant in heavy-meson chiral perturbation theory. This determinationrelies on the estimation of the residue of the form factor at the D_s^* poleand thus on an extrapolation of the form factor in the unphysical region(m_D-m_K)^2<t<(m_D+m_K)^2. We test this extrapolation for severalparametrisations of the form factors by determining the value of \hat{g}, whosevalue can be compared to other (experimental and theoretical) estimates.Several unsophisticated parametrisations, differing by the amount of physicalinformation that they embed, are shown to pass this test. An apparently moreelaborated parametrisation of form factors, the so-called z-expansion, is atvariance with the other models, and we point out some significant shortcomingsof this parametrisation for the problem under consideration.

Parametrisations of the D -> K l nu form factor and the determination of \hat{g}