  Over an interval of 120 years, the extraordinary object FG Sge has been
transformed from a hot post-AGB star to a very luminous cool supergiant.
Theoretically, this evolution has been associated with the reignition of a
helium-shell during the post-AGB stage. A series of studies of the chemical
composition of the photosphere have suggested that this evolution has been
associated with a dramatic increase of approximately 3 dex in the abundances of
s-process elements between about 1960 and 1995. The problem with this apparent
change is that it occurred at a time when the surface convection zone, which is
governed by the star's effective temperature, could not have developed
sufficiently to dredge processed material from the stellar interior to the
surface. We have reviewed the chemical evolution of FG Sge by means of
modelling the time-varying spectrum under a range of assumptions. By comparing
these models with published observational data, a self-consistent picture has
emerged. In particular we find that surface hydrogen has been depleted during
the interval in question. In contrast, the s-process abundances have generally
maintained a steady enhancement of around 1 - 2 dex, although some modest
changes may have occurred since 1950. This implies that FG Sge has not just
completed dredging up freshly-produced s-process isotopes. However, there
remains a contradiction between the observed hydrogen-depletion, the age of the
associated planetary nebula, and current evolutionary models for a pre-white
dwarf suffering a late thermal pulse.
