  Evaluation of Borelized QCD sum rules 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-light mesons on
the mass $m_q$ 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 sought $m_q$ 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 $m_q$ enables us to derive the $m_q$ dependence of the decay
constants of both pseudoscalar and vector heavy-light mesons, for which we
estimate strong isospin breaking to be of the order of 1 MeV for both charm and
beauty sectors.
