Eight-component differential equation for leptonium
  It is shown that the potential for lepton-antilepton bound states (leptonium) is the Fourier transform of the first Born approximation to the QED scattering amplitude in an 8-component equation, while 16-component equations are excluded. The Fourier transform is exact at all cms energies $-\infty < E < \infty$; the resulting atomic spectrum is explicitly CPT-invariant. 
R. Haeckl, V. Hund, H. Pilkuhn