  We use the observed $SU(3)$ breaking in the mass spectrum of mesons
containing a single heavy quark to place restrictions on the light quark
current masses. A crucial ingredient in this analysis is our recent
first-principles calculation of the electromagnetic contribution to the
isospin-violating mass splittings. We also pay special attention to the role of
higher-order corrections in chiral perturbation theory. We find that large
corrections are necessary for the heavy meson data to be consistent with $m_u =
0$.
