  We demonstrate that the generalized Gell-Mann-Low theorem permits for a
systematic expansion around the nonrelativistic limit when applied to bound
states in the Wick-Cutkosky model, Yukawa theory, and QED (in Coulomb gauge).
We apply this expansion to obtain new results for the fine and hyperfine
structure of bound states in the cases of the Wick-Cutkosky model and Yukawa
theory, and reproduce correctly the fine and hyperfine structure of hydrogenic
systems.
