  We study the classical geometry associated to fractional D3-branes of type
IIB string theory on R^4/Z_2 which provide the gravitational dual for N=2 super
Yang-Mills theory in four dimensions. As one can expect from the lack of
conformal invariance on the gauge theory side, the gravitational background
displays a repulson-like singularity. It turns out however, that such
singularity can be excised by an enhancon mechanism. The complete knowledge of
the classical supergravity solution allows us to identify the coupling constant
of the dual gauge theory in terms of the string parameters and to find a
logarithmic running that is governed precisely by the beta-function of the N=2
super Yang-Mills theory.
