J/psi D*D* vertex from QCD sum rules

We calculated the strong form factor and coupling constant for the $J/\psi D^* D^*$ vertex in a QCD sum rule calculation. We performed a double Borel sum rule for the three point correlation function of vertex considering both $J/\psi$ and $D^*$ mesons off--shell. The form factors obtained are very different, but they give the same coupling constant.