  We present recent progress in the lattice calculation of leptonic decay
constants for $B_{(s)}$ and $D_{(s)}$ mesons using the Oktay-Kronfeld (OK)
action for charm and bottom valence quarks, whose masses are tuned
non-perturbatively. The calculations are done on 6 HISQ ensembles generated by
the MILC collaboration with $N_f=2+1+1$ flavors. We also use the HISQ action
for the light spectator quarks. Results are presented for the ratios
$f_{B_s}/f_B$ and $f_{D_s}/f_D$, which reflect $SU(3)$ flavor symmetry
breaking, and are independent of the renormalization constants of the axial
currents.
