  The isospin mass splittings of the pseudoscalar and vector D and B
light-heavy quark system have been calculated using the method of QCD sum
rules. Nonperturbative QCD effects are shown to be very small, so that mass
splittings arise almost completely from current quark mass splitting and
electromagnetic effects, for which a new gauge invariant QED treatment is used.
The results are consistent with experiment. A measurement of the isospin
splitting of the vector B mesons would give valuable information about quark
mass splittings.
