Making manifest the symmetry enhancement for coincident BPS branes
  We consider g coincident M-5-branes on top of each other, in the KK monopole background Q of multiplicity N. The worldvolume of each M-5-brane is supposed to be given by the local product of the four-dimensional spacetime and an elliptic curve. In the coincidence limit, all these curves yield a single (Seiberg-Witten) hyperelliptic curve, while the gauge symmetry is enhanced to U(N). We make this gauge symmetry enhancement manifest by considering the hypermultiplet LEEA which is given by the spacetime N=2 non-linear sigma-model (NLSM) having Q as the target space. The hyper-K"ahler manifold Q is given by the multicentre Taub-NUT space, which in the coincidence limit amounts to the multiple Eguchi-Hanson (ALE) space Q. The NLSM is most naturally described in terms of the hyper-K"ahler coset construction on SU(N,N)/U(N) in harmonic superspace, by using the auxiliary (in classical theory) N=2 vector superfields as Lagrange multipliers, with FI terms resolving the singularity. The Maldacena limit, in which the hypermultiplet LEEA becomes extended to the N=4 SYM with the gauge group U(N), arises in quantum field theory due to a dynamical generation of the N=2 vector and hypermultipet superfields, when sending the FI terms to zero. 
Sergei V. Ketov (ITP, University of Hannover, Germany)