  We extract the strong coupling constants of three mesons, each of which is
composed of either two charm quarks or one charm quark and one light (i.e.,
$u$, $d$, or $s$) quark, from the matrix elements for the transitions of two of
these mesons induced by appropriate quark currents within the framework of a
relativistic dispersion approach to the constituent-quark picture of mesons.
Among others, we also analyse the impact of the violation of the SU(3) flavour
symmetry by the quark masses. In the case of mesons containing one light quark,
we observe, in two respects, discrepancies between our findings and the
predictions of QCD sum rules: our strong couplings exceed considerably the ones
emerging from QCD sum rules, and, in our approach, the replacement of a light
quark by a strange quark entails, in contrast to QCD sum rules, a reduction of
the magnitudes of the strong couplings.
