Pion and thermal photon spectra as a possible signal for a phase transition

We calculate thermal photon and neutral pion spectra in ultrarelativistic heavy-ion collisions in the framework of three-fluid hydrodynamics. Both spectra are quite sensitive to the equation of state used. In particular, within our model, recent data for $S+Au$ at $200$ AGeV can only be understood if a scenario with a phase transition (possibly to a quark-gluon plasma) is assumed. Results for $Au+Au$ at $11$ AGeV and $Pb+Pb$ at $160$ AGeV are also presented.