  Masses and decay constants of mesons containing a single $c$ or $b$ quark are
described within the framework of heavy-quark symmetry. The $B_s^* - B_s$ and
$\bar B^{*0} - \bar B^0$ mass splittings are found equal to within a fraction
of an MeV. Decay constants of $D$ and $B$ mesons are estimated using isospin
mass splittings in the $D$, $D^*$, $B$, and $B^*$ states to isolate the
electromagnetic hyperfine interaction between quarks. A relation following from
the use of splittings in kaons is also considered.
