Supersymmetry enhancement and junctions in S-folds

We study supersymmetry enhancement from N = 3 to N = 4 proposed by Aharony and Tachikawa by using string junctions in S-folds. The central charges carried by junctions play a central role in our analysis. We consider planer junctions in a specific plane. Before the S-folding they carry two complex central charges, which we denote by Z and (Z) over bar. The S-fold projection eliminates (Z) over bar as well as one of the four supercharges, and when the supersymmetry is enhanced (Z) over bar should be reproduced by some non-perturbative mechanism. For the models of Z(3) and Z(4) S-folds which are expected to give SU(3) and SO(5) N = 4 theories we compare the junction spectra with those in perturbative brane realization of the same theories. We establish one-to-one correspondence so that Z coincides. By using the correspondence we also give an expression for the enhanced central charge (Z) over bar.