$D^* D_s K$ and $D_s ^* D K$ vertices in a QCD Sum Rule approach
  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%. 
M.E. Bracco, A. Cerqueira Jr., M. Chiapparini, A. Lozea and M. Nielsen