Evaluation of Borelized QCD sumrules in the so-called local-duality limit of infinitely large Borel mass parameter provides an alternate route for extraction of the dependence of the decay constants of heavy–lightmesons on themass mq of the involved light quark q: For appropriate choices of the two-point correlation functions of currents interpolating the hadrons under study, the local-duality limit forces all nonperturbative contributions parametrized by vacuum condensates to such kind of correlator to vanish. As a consequence, the soughtmq dependence of the heavy–light meson decay constants proves to be controlled primarily by the correlator contributions from perturbative QCD. Our knowledge of the analytic behaviour of the latter as functions of mq enables us to derive the $m_q$ dependence of the decay constants of both pseudoscalar and vector heavy–lightmesons, for which we estimate strong isospin breaking to be of the order of 1 MeV for both charmand beauty sectors.