  The $B_c ^+$ family of $(c\bar{b})$ mesons with beauty and charm is of
special interest among heavy quarkonium systems. The $B_c ^+$ mesons are
intermediate between $(c\bar{c})$ and $(b\bar{b})$ states both in mass and
size, so many features of the $(c\bar{b})$ spectrum can be inferred from what
we know of the charmonium and bottomonium systems. The unequal quark masses
mean that the dynamics may be richer than a simple interpolation would imply,
in part because the charmed quark moves faster in $B_c$ than in the $J/\psi$.
Close examination of the $B_c ^+$ spectrum can test our understanding of the
interactions between heavy quarks and antiquarks and may reveal where
approximations break down. ...
