We discuss the leptonic decay constants of heavy–light mesons by means of Borel QCD sum rules in the local-duality (LD) limit of infinitely large Borel mass parameter. In this limit, for an appropriate choice of the invariant structures in the QCD correlation functions, all condensate contributions vanish and all nonperturbative effects are contained in only one quantity, the effective threshold. We study properties of the LD effective thresholds in the limits of large heavy-quark mass mQ and small light-quark mass mq. We show that the dependence of the meson decay constants on mq arises predominantly (at the level of 70–80%) from the calculable mq-dependence of the perturbative spectral densities. Making use of the lattice QCD results for the decay constants of nonstrange and strange pseudoscalar and vector heavy mesons, we obtain solid predictions for the decay constants of heavy–light mesons as functions of mq in the range from a few to 100 MeV and evaluate the corresponding strong isospin-breaking effects: fD+ − fD0 = (0.96 ± 0.09) MeV, fD∗+ − fD∗0 = (1.18 ± 0.35) MeV, fB0 − fB+ = (1.01 ± 0.10) MeV, fB∗0 − fB∗+ = (0.89 ± 0.30) MeV.