  We calculate the strong form factors and coupling constants of $ D^* D_s K$
and $D_s^* D K$ vertices using the QCD sum rules technique. In each case we
have considered two different cases for the off-shell particle in the vertex:
the ligthest meson and one of the heavy mesons. The method gives the same
coupling constant for each vertex. When the results for different vertices are
compared, they show that the SU(4) symmetry is broken by around 40%.
