ON THE FORM-FACTORS OF THE D-S(-]PHI-MU(+) NU(MU) DECAY())

We apply the infinite mass effective theory, when a heavy quark mass t ends to infinity, and chiral perturbation theory at the quark level, b ased on the extended Nambu-Jona-Lasinio model with linear realization of chiral U(3) x U(3) symmetry, to calculate the form factors of the D -s(+) --> phi mu(+)nu(mu), decay up to the first order in current s-qu ark mass. The theoretical results are compared with experimental data and found to be in good agreement.