vertex from QCD sum rules

The form factors and the coupling constant of the vertex are calculated using the QCD sum rules method. Three-point correlation functions are computed considering both K and B mesons off-shell and, after an extrapolation of the QCDSR results, we obtain the coupling constant of the vertex. We study the uncertainties in our result by calculating a third form factor obtained when the is the off-shell meson, considering other acceptable structures and computing the variations of the sum rulesʼ parameters. The form factors obtained have different behaviors but their simultaneous extrapolations reach to the same value of the coupling constant . We compare our result with other theoretical estimates.