  We study the effects of an arbitrary external perturbation in the statistical
properties of the S-matrix of quantum chaotic scattering systems in the limit
of isolated resonances. We derive, using supersymmetry, an exact
non-perturbative expression for the parameter dependent autocorrelator of two
S-matrix elements. Universality is obtained by appropriate rescaling of the
physical parameters. We propose this universal function as a new signature of
quantum chaos in open systems.
