Event-by-event fluctuations of particle ratios in central Pb+Pb collisions at 20-158 A GeV

In the vicinity of the QCD phase transition, critical fluctuations have been predicted to lead to non-statistical fluctuations of particle ratios, depending on the nature of the phase transition. Recent results of the NA49 energy scan programme show a sharp maximum of the ratio of K+ to pi(+) yields in central Pb+Pb collisions at beam energies of 20-30 A GeV. This observation has been interpreted as an indication of a phase transition at low SPS energies. We present first results on event-by-event fluctuations of the kaon to pion and proton to pion ratios at beam energies close to this maximum.