On ortho-positronium and gauge
  Binding energy of the $1^-$ state (ortho-positronium) in QED is calculated using the one-photon exchange Bethe-Salpeter equation in the Feynman and Coulomb gauges for different coupling constants $\alpha$. Calculations show there is a remarkable difference in values of the binding energy for different coupling constants in these two gauges. 
G.V.Efimov (Laboratory of Theoretical Physics, Joint Institute for   Nuclear Research)