Branes and Electric-Magnetic Duality in Supersymmetric QCD
  We study the Type IIA limit of the M theory fivebrane configuration corresponding to N=1 supersymmetric QCD with massless quarks. We identify the effective gauge coupling constant that fits with Novikov-Shifman-Veinshtein-Zakharov exact beta function. We find two different Type IIA limits that correspond to the electric and magnetic descriptions of SQCD, as observed in the massive case by Schmaltz and Sundrum. The analysis is extended to the case of symplectic and orthogonal gauge groups. In any of the cases considered in this paper, the electric and magnetic configurations are smoothly interpolated via $M$ theory. This is in sharp contrast with the proposed derivation of N=1 duality within the weakly coupled Type IIA string theory where a singularity is inevitable unless one turns on a parameter that takes the theory away from an interesting point. 
Kentaro Hori (UC Berkeley/LBNL)