Infrared photometry of Sakurai's object (V4334 Sgr) in 1996-1999

The infrared photometric observations of V4334 Sgr in 1996-1999 are presented. Together with optical data, they have allowed us to accurately estimate the bolometric flux from this star and to investigate the structure of its dust envelope over the above period. The star is shown to have passed through four well-defined stages in these four years as it moved backward along the post-AGE track, and it now appears to have started moving forward after a halt. At the first stage (1996), there was no dust in the star's envelope. Its visual brightness slightly increased, and it reddened in the entire observed spectral range. The bolometric flux also gradually rose. At the second stage (1997), an optically thick dust envelope condensed around the star, which, however, essentially did not manifest itself at optical wavelengths. The bolometric flux continued to rise through an increase in the star's infrared brightness alone; the rate of its rise also increased. At the third stage (1998-March 1999), V4334 Sgr entered the R CrB phase. First two shallow minima and then two deep minima were observed at optical wavelengths. The star appreciably reddened during the deep minima. The bolometric flux ceased to rise and began to gradually fall in the second half of 1998. At the fourth stage (since March 1999 up until now), V4334 Sgr has been at a protracted deep minimum, which is atypical of the R CrB stars. The bolometric flux between March and October underwent no significant variations. We describe the structure of the dust envelope around V4334 Sgr since its formation. From June 1997 until July 1998, the optical depth of the dust shell, its inner and outer radii, and its mass increased by factors of similar to 2.2, similar to 2.0, 2.3, and similar to 10, respectively. In July 1998, tau(V) approximate to 2.3, R-d,R-in = 7.4 x 10(14) cm, R-d,R-in/R-d,R- out approximate to 0 7 (R-d,R- in/R* approximate to 47), and M-dust approximate to 1 6 x 10(-7) Mg. (C) 2000 MAIK "Nauka/Interperiodica ".