then steadily cooled, with the spectral class becoming as late as K2 in the 1980s. The spectral class has since stayed as a G or K type supergiant, but there have been dramatic changes. The abundances of various elements have either increased or decreased: s-process elements became at least 25 times more abundant between 1967 and 1974; iron peak elements became less visible; and carbon-rich dust became strongly visible after 1992. Observations of the spectrum after 1992 are hindered by the dust formation, but the s-process and rare earth elements appear to have continued becoming more abundant. Planetary nebula There is a very faint visible planetary nebula, Henize 1-5, around FG Sagittae, around visual magnitude 23. This formed when FG Sagittae first left the asymptotic giant branch. FG Sagittae is now losing mass at about every million years and a dust shell has formed around the star. This may form a second planetary nebula. Evolution The effective temperature of FG Sagittae in 1930 would have been about , possibly as hot as in 1890, then cooling to about by 1975. Detailed analysis of the spectral energy distribution during the 1980s show a slow decrease in temperature to as low as . During the deep fades since 1992, even lower temperatures have been calculated, but these may represent observations of obscuring dust rather than the surface of the star itself. The bolometric luminosity of FG Sagittae increased steadily from around at the end of the 19th century to over by 1965. The luminosity then became more or less stable until 1992. As the star cooled and became more luminous, its radius increased from around in 1900 to about by 1992. When the star faded in 1992, it was obscured by dust formation and comparisons of temperature and luminosity became more difficult. The visual luminosity dropped by about five magnitudes, but the infrared brightness increased by a comparable amount. Models of the dust around the star suggest that the luminosity dropped sharply for a few hundred days as dust formed and was heated, but the underlying stellar luminosity was essentially constant and remained constant until at least 2001. The underlying properties of FG Sagittae changed on a timescale almost unheard-of for a star, from a small very hot post-asymptotic giant branch star becoming a white dwarf, to a hot supergiant and then a cool supergiant. This is believed to have been due