  In this paper, the low-mass electron pair production in 158 AGeV/c Pb-Au
collisions is investigated with the Cherenkov Ring Electron Spectrometer
(CERES) at the Super Proton Synchrotron accelerator (SPS) at CERN. The main
goal is to search for modifications of hadron properties in hot and dense
nuclear matter. The presented re-analysis of the 1996 data set is focused on a
detailed study of the combinatorial-background subtraction by means of the
mixed-event technique. The results confirm previous findings of CERES. The
dielectron production in the mass range of 0.25<m(ee)<2GeV/c**2 is enhanced by
a factor of 3.0+-1.3(stat.)+-1.2(syst.) over the expectation from neutral meson
decays. The data is compared to transport model calculations and seem to favor
the version including in-medium effects.
  Furthermore, the development of a new technology to manufacture
ultralightweight mirrors for Ring Imaging Cherenkov detectors (RICH) is
described. Replacement of the RICH-2 glass mirror by a mirror almost
transparent to electrons would considerably improve the performance of the
upgraded CERES detector system including a radial Time Projection Chamber
(TPC).
