1995A&A...294..453V

We compare the observed evolutionary and pulsational characteristics of the variable post-AGB star FG Sge with theoretical models. Therefore, we had to reconstruct the evolutionary track on the theoretical HR diagram from the apparent magnitudes for the years 1890-1992. The variability of FG Sge was investigated including data from the early Sonneberg photographic monitoring campaigns. It appears that the reconstructed evolutionary track, from presumably spectral type O3 around 1880 to K2 at about 1992 runs slightly steeper on the Hertzsprung-Russell diagram than what stellar evolution canonically shows for post-AGB stars after a late He-shell flash. A period analysis applied to the photographic observations performed in the 1930s revealed FG Sge to have been pulsating (P≃5d) already when the temperature was of the order of 20000K. Theoretical stability analyses of appropriate stellar envelopes agree well with such findings. The best fitting of theoretical models with the observed pulsational characteristics requires M=0.8M_☉ and logL/L_☉=4.08 which corresponds to a distance of 3.9kpc. The deep fading and the simultaneous blue colors are attributed to the formation of a dense circumstellar dust cloud in front of the star and by scattering of the stellar light by parts of the cloud. A causal connection between the expulsion of the cloud and the decay of the pulsation is not inconceivable.