Upper bounds on fD and fDs from two-point correlation function in QCD

The correlation function of two pseudoscalar charmed quark currents with a positive hadronic spectral density is employed to obtain upper bounds on the decay constants of D and Ds mesons. Including all known terms of the operator product expansion of this correlation function in QCD and taking into account the estimated uncertainties, we obtain fD<230 MeV and fDs<270 MeV. Comparison with the decay constants determined from D→lνl and Ds→lνl measurements reveals a tension between the bound and current experimental value of fDs.