Possible open channel effects in the charmonium spectrum

QCD sum rules are used to give absolute bounds for the semi leptonic decay widths of the heavy mesonsD ⁺ andF ⁺ into η and η′. 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 theF ⁺ decays may be important in relative value (if one relies on the present estimate of theF ⁺ lifetime), the small fraction (less than 1%) obtained in theD ⁺ 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.