  Employing NEXUS, one of the most recent simulation programs for heavy ions
collisions, we investigate in detail the Hanbury-Twiss correlation function for
charged pions for reactions 158 GeV Pb+Pb. For this study we supplement the
standard simulation program by electromagnetic interactions. We find that the
string fragmentation introduces strong space-momentum correlations and the
freeze out times of the correlated pairs have a wide distribution. Both has a
strong influence on the apparent source radius determined from the measured
correlation correlation function.An equally strong influence is observed for
the final state electromagnetic interaction.
