DEDUCTIONS FROM THE RECONSTRUCTED EVOLUTIONARY AND PULSATIONAL HISTORY OF FG SAGITTAE

We compare the observed evolutionary and pulsational characteristics o f the variable post-AGE star FG Sge with theoretical models. Therefore , we had to reconstruct the evolutionary track on the theoretical HR d iagram from the apparent magnitudes for the years 1890 - 1992. The var iability of FG Sge was investigated including data from the early Sonn eberg photographic monitoring campaigns.It appears that the reconstruc ted evolutionary track, from presumably spectral type O3 around 1880 t o K2 at about 1992 runs slightly steeper on the Hertzsprung-Russell di agram than what stellar evolution canonically shows for post-AGE stars after a late He-shell flash. A period analysis applied to the photogr aphic observations performed in the 1930s revealed FG Sge to have been pulsating (P approximate to 5 d) already when the temperature was of the order of 20 000 K. Theoretical stability analyses of appropriate s tellar envelopes agree well with such findings. The best fitting of th eoretical models with the observed pulsational characteristics require s M = 0.8M. and logL/L. = 4.08 which corresponds to a distance of 3.9 kpc. The deep fading and the simultaneous blue colors are attributed t o the formation of a dense circumstellar dust cloud in front of the st ar and by scattering of the stellar light by parts of the cloud. A cau sal connection between the expulsion of the cloud and the decay of the pulsation is not inconceivable.