The deep photometric minimum of FG Sagittae in 1998 as a typical manifestation of the R CrB phase inthe post-AGB stellar evolution

We performed photometric BVRI observations of FG Sge during 47 nights in May-October 1998. The star exhibited a sixth deep minimum after the onset of the R CrB phase in 1992. It was a double minimum, as that in 1994; the star experienced a bluing on the descent to the first minimum, as reflected by B-V and, less dramatically, by V-R, and reddened on the descent to the second minimum. As at the previous minima, the bluing of the star is attributable to the formation of small, light-scattering dust grains. In contrast to the previous minima, the blue minimum was out of phase with the pulsation minimum, while the maximum of the second variation occurred at the pulsation minimum. Spectroscopic observations with resolution of about 5 A have revealed the appearance of emission in the NaI doublet lines and the Swan bands in May and its enhancement as the star faded. The NaI emission line had been visible until October 1998, while the Swan bands were in absorption in the fall. We have found evidence for self-reversal of the entire stellar spectrum on the descent to minimum light. The duration of the self-reversal phase increases with time, which may be caused by an increase in the sizes of the circumstellar gas envelope produced by the stellar wind. The behavior of FG Sge at the 1998 minimum, as at the five previous minima, is typical of the R CrB stars.