  We point out that formation time of pions produced in heavy ion collisions
modifies the mass spectrum of dileptons produced via pipi -> ee annihilation.
Increasing formation time enhances the production of dileptons with lower
masses. The effect offers an explanation of a part of the enhanced production
of dileptons below the rho-meson mass as observed by the CERES and HELIOS
Collaborations at the CERN SPS.
