  The partial width of the decay D^*+_s -> D^+_s pi^0 going through the
violation of isotopical symmetry is computed with respect to the partial width
of the radiative decay D^*+_s -> D^+_s gamma in the effective quark model with
chiral U(3)xU(3) symmetry incorporating Heavy quark effective theory (HQET) and
Chiral perturbation theory at the quark level (CHPT)_q. We investigate a
sensitivity of the ratio of the partial widths R_0 to current s-quark mass
corrections and find a strong dependence.
