We explore, by a rather peculiar limit of QCD sum rules subject to a Borel transformation (realized by allowing the related Borel mass parameter to approach infinity), how the small difference of the masses of up and down quark translates into the leptonic decay constants of pseudoscalar or vector heavy–light mesons. For the charmed and bottom mesons, we find that the decay constants of their
lowest-lying charged and neutral representatives should differ by an amount of the order of 1 MeV.