Radiative Corrections to P-Levels in the Two-Body QED Problem
  The physical origin of the $m\alpha^5$ radiative corrections to $P$-levels in the two-body QED problem is elucidated. Then we demonstrate that the next order, $m\alpha^6$, corrections to those levels are due to the anomalous magnetic moment only. 
M. I. Eides, I. B. Khriplovich and A. I. Milstein