Assuming that in the asymptotic region of large $S$, the ${\pi }_{l3\mathrm{}}$ and $K_{l3\mathrm{}}$ form factors $f\left(S\right)\mathrm{}$ and $F_{+}\mathrm{}\left(S\right)\mathrm{}$ are related by the SU(3) symmetry, a superconvergence relation is suggested, from which, on saturation by the $\rho$ and the $K^{*}$, we obtain the approximate result $F_{+}\mathrm{}\left(0\right)=-\frac{\left(\frac{1}{\sqrt{2}}\right){M_{\rho }}^2}{{M_{K^{*}}}^2}$.

• Received 23 May 1967

DOI:https://doi.org/10.1103/PhysRevLett.19.263