  The common interpretation of HBT data measured at top SPS energies leads to
apparent source lifetimes of 6-8 fm/c and emission duration of approximately
2-3 fm/c. We investigate a scenario with continuous pion emission from a
long-lived (~17 fm/c) thermalized source in order to show that it is not
excluded by the data. Starting from a description of the source's spacetime
expansion based on gross thermodynamical properties of hot matter (which is
able to describe a number of experimental observables), we introduce the pion
emission function with a contribution from continuous emission during the
source's lifetime and another contribution from the final breakup and proceed
by calculating the HBT parameters R_out and R_side. The results are compared
with experimental data measured at SPS for 158 AGeV central Pb-Pb collisions.
We achieve agreement with data, provided that some minor modifications of the
fireball evolution scenario are made and find that the parameter R_out is not
sensitive to the fireball lifetime, but only to the duration of the final
breakup, in spite of the fact that emission takes place throughout the whole
lifetime. We explicitly demonstrate that those findings do not alter previous
results obtained within this model.
