  We show that the dilepton spectra measured by the CERES collaboration in Pb +
Au interactions for various charge multiplicities can be reproduced by a
hadronic initial state with reduction in the masses of the vector mesons in the
thermal bath. Though such an effect may also be achieved by a large broadening
of the spectral function we show that the photon spectra is insensitive to
this. It is found that for higher multiplicities a good description of the data
can also be obtained with quark gluon plasma initial state if the reduction of
the vector meson masses in the mixed and hadronic phases is taken into account.
We observe that a thermal source with initial temperature ~ 200 MeV can
reproduce the observed enhancement in the low mass region of the dilepton
spectra. It is not possible to state which one of the two initial states (QGP
or hadronic) is compatible with the data. These findings are in agreement with
our earlier results obtained from the analysis of the WA98 photon spectra. We
estimate the number of pion-pion collisions near the rho-peak of the dilepton
spectra and argue that thermal equilibrium may have been achieved in the
system, justifying the use of hydrodynamical model to describe the space time
evolution.
