  The $J/\psi D D$ form factor is evaluated in a QCD sum rule calculation for
both $D$ and $J/\psi$ off-shell mesons. We study the double Borel sum rule for
the three point function of two pseudoscalar and one vector meson current. We
find that the momentum dependence of the form factors is different if the $D$
or the $J/\psi$ meson is off-shell, but they lead to the same coupling constant
in the $J/\psi D D$ vertex.
