Chiral Perturbation Theory for $f_{D_S}/f_D$ and $B_{B_S}/B_{B}$
  The decay constants for the $D$ and $D_S$ mesons, denoted $f_D$ and $f_{D_S}$ respectively, are equal in the $SU(3)_V$ limit, as are the hadronic amplitudes for $B_S-\bar B_S$ and $B^0-\bar B^0$ mixing. The leading $SU(3)_V$ violating contribution to $\left( f_{D_S} / f_D \right)$ and to the ratio of hadronic matrix elements relevant for $B_S-\bar B_S$ and $B^0-\bar B^0$ mixing amplitudes are calculated in chiral perturbation theory. We discuss the formalism needed to include both meson and anti-meson fields in the heavy quark effective theory. 
B. Grinstein, E. Jenkins, A.V. Manohar, M.J. Savage and M.B. Wise