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 $f_{D_s}/f_D$ as input, one can predict quantitatively the ratios of $f_{D_s^*}/f_{D^*}$, $f_{B_s}/f_B$, and $f_{B^*_s}/f_{B^*}$ taking into account heavy-quark spin-flavor symmetry breaking effects on the relevant low-energy constants. The predicted relations between these ratios, $f_{D^*_s}/f_{D^*}<f_{D_s}/f_D$ and $f_{B_s}/f_B>f_{D_s}/f_D$, 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.