INFRARED OBSERVATIONS OF FG SAGITTAE IN 1993-1997

We present the infrared JHKLM photometry of the peculiar variable FG S ge in 1993-1997. The deep minima observed in the optical light curve o f FG Sge since 1992 are shown to result each time from the condensatio n of a dust layer in the envelope of the star, which is accompanied by a relatively long (similar to 1.5 yr) decline in its mean bolometric luminosity. The amplitude of its pulsations with a period P approximat e to 115 days remains at the level of the 1980s. The formation of dust embryos, which then rapidly (in similar to 30 days) grow to sizes sim ilar to 0.1 mu m, takes similar to 40 days and appears to be concentra ted in isolated fragments (clumps, clouds) of the corresponding gaseou s layer As a result, profound dimmings of FG Sge are not accompanied b y its appreciable reddening. The subsequent reddening is attributable to the appearance in the envelope of finer dust grains whose mass frac tion does not exceed 20%, however. We construct a model for FG Sge as a star of variable luminosity surrounded by a dust envelope in the for m of a layer cake with a new dust layer emerging during each deep mini mum. This model reproduces the main features of the star's behavior be ginning from its first deep minimum in 1992. We show that the dust env elope must contain small (0.05 mu m), medium-size (0.1 mu m), and larg e (0.25 mu m) graphite grains whose relative number varies with time. The masses of the three dust layers formed in 1992 and 1994 lie within the range (1-4) x 10(-8) M., and their condensation occurred at a dis tance of 40-90 R- from the star.