Applying the QCD sum-rule machinery in the so-called local-duality shape to heavy-light mesons reveals that, as a consequence of the non-zero mass gap between up and down quarks, the leptonic decay constants of the neutral and charged versions of the $D,$ $D^\ast,$ $B$ and $B^\ast$ mesons differ by approximately 1 MeV.Comment: 7 pages, 5 figures, talk presented at "QCD@Work 2018 - International Workshop on Quantum Chromodynamics: Theory and Experiment" (25 - 28 June 2018, Matera, Italy
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