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The background field method for N = 2 super Yang-Mills theories in harmonic superspace is developed. The ghost structure of the theory is investigated. It is shown that the ghosts include two fermionic real ω-hypermultiplets (FaddeevPopov ghosts) and one bosonic real ω-hypermultiplet (Nielsen-Kallosh ghost), all in the adjoint representation of the gauge group. The one-loop effective action is analysed in detail and it is found that its structure is determined only by the ghost corrections in the pure super Yang-Mills theory. As applied to the case of N = 4 super Yang-Mills theory, realized in terms of N = 2 superfields, the latter result leads to the remarkable conclusion that the one-loop effective action of the theory does not contain quantum corrections depending on the N = 2 gauge superfield only. We show that the leading low-energy contribution to the one-loop effective action in the N = 2 SU (2) super Yang-Mills theory coincides with Seiberg's perturbative holomorphic effective action.

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We present, in the N = 2, D = 4 harmonic superspace formalism, a general method for constructing the off-shell effective action of an N = 2 abelian gauge superfield coupled to matter hypermultiplets. Using manifestly N = 2 supersymmetric harmonic supergraph techniques, we calculate the low-energy corrections to the renormalized one-loop effective action in terms of N = 2 (anti)chiral superfield strengths. For a harmonic gauge prepotential with vanishing vacuum expectation value, corresponding to massless hypermultiplets, the only non-trivial radiative corrections to appear are non-holomorphic. For a prepotential with non-zero vacuum value, which breaks the U (1)-factor in the N = 2 supersymmetry automorphism group and corresponds to massive hypermultiplets, only non-trivial holomorphic corrections arise at leading order. These holomorphic contribution are consistent with Seiberg's quantum correction to the effective action, while the first non-holomorphic contribution in the massless case is the N = 2 supersymmetrization of the Heisenberg-Euler effective Lagrangian.