The remarkable evolution of the post-AGB star FG Sge.

FG Sagittae is one of the most important key objects for post-AGB stellar evolutionary studies. As a consequence of a final helium shell flash, this unique variable has shown real evolutionary changes on human timescales during this century. The recently observed variations in the emission lines of the surrounding old planetary nebula prove that there is no hot exciting object hidden inside the nebula, thus there is only one central star, namely FG Sge itself. Consequently, we are witnessing the rapid evolution of a single star. Turning to an R CrB type variable in the early 90s, FG Sge also solved the question of the evolutionary state of this type of stars. Some aspects of the observational history of FG Sge are reviewed in this paper and compared with predictions from model calculation results.