  We compute the one-loop holomorphic effective action of the massless Cartan
sector of N=2 SYM theory in the Coulomb branch, taking into account the
contributions both from the charged hypermultiplets and off-diagonal components
of the gauge superfield. We use the manifestly supersymmetric harmonic
superfields diagram techniques adapted to N=2 supersymmetry with the central
charges induced by Cartan generators. The (anti)holomorphic part proves to be
proportional to the central charges and it has the generic form of Seiberg's
action obtained by integrating U(1) R-anomaly. It vanishes for N=4 SYM theory,
i.e. the coupled system of N=2 gauge superfield and hypermultiplet in the
adjoint representation.
