The enhancon and N=2 gauge theory/gravity RG flows

We study the family of ten-dimensional type-IIB supergravity solutions corr esponding to renormalisation group flows from N = 4 to N = 2 supersymmetric SU(N) Yang-Mills theory. Part of the solution set corresponds to a submani fold of the Coulomb branch of the gauge theory, and we use a D3-brane probe to uncover details of this physics. At generic places where supergravity i s singular, the smooth physics of the probe yields the correct one-loop for m of the effective low energy gauge coupling. The probe becomes tensionless on a ring at finite radius. Supergravity flows which end on this "enhancon " ring correspond to the vacua where extra massless degrees of freedom appe ar in the gauge theory, and the gauge coupling diverges there. We identify an SL(2, Z) duality action on the enhancon ring which relates the special v acua, and comment on the massless dyons within them. We propose that the su pergravity solution inside the enhancon ring should be excised, since the p robe's tension is unphysical there.