  In heavy ion collisions resonances can create strong non-Gaussian effects in
the 2-pion correlation data. Hence, the commonly used Gaussian fit parameters
do not fully characterize these correlators. We suggest a different set of HBT
parameters which does not presuppose a particular shape of the correlator and
allows to extract additional (non-Gaussian) information. Within a simple model
for an expanding source including resonance decays it is shown that this
additional information provides a clean distinction between scenarios with and
without transverse flow.
