  Consequences resulting from the D Dbar excess derived indirectly by the NA50
experiment in S+U and Pb+Pb collisions at sqrt(s)=19, 17 GeV, relevant for the
identification of the QCD phase transition in these collisions, are discussed.
The dependence of open and closed charm yields in Pb+Pb collisions on the
number of participating nucleons (N) indicates non thermal charm production and
J/Psi dissociation, stronger than the absorption seen in any other elementary
hadron. The J/Psi in central Pb+Pb collisions could originate dominantly from c
cbar pair coalescence out of a hadronizing quark and gluon environment.
Furthermore, the J/Psi appears to be suppressed in S+U collisions at sqrt(s)=19
GeV, as opposed to current interpretations. A significant change in the
(J/Psi)/D Dbar ratio as well as in the number density of kaons is observed
above energy density approx. 1 GeV/fm^3, suggesting a change of phase at this
energy density, and underlining the importance of direct open charm
measurements.
