QCD sum rules and I=O final state in D+ and F+ mesons semi-leptonic decays

QCD sum rules are used to give absolute bounds for the semi leptonic decay widths of the heavy mesons D+ and F+ into eta and eta'. This method, very stable and reliable in those cases where a heavy mass scale is involved, does not rely on any quark gluon picture of the above decays. While the F+ decays may be important in relative value (if one relies on the present estimate of the F+ lifetime), the small fraction (less than 1%) obtained in the D+ case, where only the annihilation mechanism can be advocated, is in agreement with what one expects for a Cabibbo suppressed decay, and disfavors a special quantitative enhancement due to this particular mechanism. (orig.)