2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/222996The 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 $D_s$ 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 $f_{D}<230 $ MeV and $f_{D_s}<270$ MeV. Comparison with the decay constants determined from $D\to l ν_l$ and $D_s\to l ν_l$ measurements, reveals a tension between the bound and current experimental value of $f_{D_s}$.8 pages, 3 figures, a few comments,a table and references added, version to appear in Phys.Rev.DHigh Energy Physics - Phenomenologytext