SU(3) breaking corrections to the D, D ^*, B, and B ^* decay constants

We report on a first next-to-next-to-leading order calculation of the decay constants of the D (D ^*) and B (B ^*) mesons using a covariant formulation of chiral perturbation theory. It is shown that, using the state-of-the-art lattice QCD results on fDs/fD as input, one can predict quantitatively the ratios of fDs*/fD*, fBs/fB, and fBs*/fB* taking into account heavy-quark spin-flavor symmetry breaking effects on the relevant low-energy constants. The predicted relations between these ratios, fDs*/fD*<fDs/fD and fBs/fB>fDs/fD, and their light-quark mass dependence should be testable in future lattice QCD simulations, providing a stringent test of our understanding of heavy quark spin-flavor symmetry, chiral symmetry and their breaking patterns.