Heavy-Meson Decay Constants: QCD Sum-Rule Glance at Isospin Breaking
  QCD sum rules for decay constants of heavy mesons with $u$ or $d$ quark yield for $B$ mesons much less isospin breaking than lattice QCD but good agreement for $D$ mesons. 
Wolfgang Lucha, Dmitri Melikhov, Silvano Simula
hep-ph hep-lat nucl-th