COMMISSIONS 27 AND 42 OF THE IAU
              INFORMATION BULLETIN ON VARIABLE STARS
                          Number 3959

                                                  Konkoly Observatory
                                                  Budapest
                                                  7 December 1993

                                                  HU ISSN 0374 - 0676


           FG SAGITTAE HAS BEEN A CARBON STAR SINCE 1981



 Numerous works have been made on the post-AGB peculiar variable star FG Sge.
Its spectral type changed from B4 I in 1955 to A5 Ia in 1967 then F6-7I in 1980 (e.g.
Langer et al. 1974; Montesinos et al. 1990, and references therein). The luminosity in the
optical region started to decline suddenly in August 1992 (e.g. Papousek 1992; Guinan
et al. 1992; Jurcsik 1992). The spectral variation of this object has been monitored at
Asiago Observatory since 1979 using the prismatic spectrograph Camera VI with RCA
S-20 image tube mounted on the 120 cm reflector. The spectral range is from 380 to 770
nm, and the resolution is about 0.4 nm at H_gamma. The spectra were digitized using the PDS
microdensitometer of the Astronomical Observatory of Padua. Log-intensity tracings of
some selected spectra are shown in Fig. 1, where the ordinate is an arbitrary scale.

                                     [FIGURE 1]

  Figure 1.  Log-intensity tracings of spectra of FG Sge taken on (from top to
bottom) 1981 Nov. 17, 1982 Nov. 17, 1983 Aug. 20, 1985 Sept. 10, and 1986 Oct. 2

 The first clear profiles of the C2-Swan bands at 473.7 and 516.5 nm were detected on
the spectra taken in November 1981 (Fig. 1). These bands were prominent in 1982 and
1985, while were weak in 1983 and 1986 (Fig. 1). They were again prominent in 1988 and
in 1992 before the beginning of the light decline (Iijima et al. in preparation). It may be
obvious that FG Sge has been a carbon star, but with a very unstable atmosphere, in the
last decade.
 The sudden light decline of this object in 1992 may have been due to, as suggested by
several astronomers (e.g. Guinan et al. 1992; Iben & Livio 1993; Stone et al. 1993; 
Woodward et al. 1993), a condensation of carbon dust which is not a rare phenomenon in the
atmosphere of hydrogen deficient carbon stars (Warner 1967). This scenario is consistent
with the increasing of the infrared flux and the lack of the emission feature of silicate dust
observed by Woodward et al. (1993). Since FG Sge ejected the planetary nebula He 1-5
about 6000 years ago (e.g. Flannery & Herbig 1973), its atmosphere could be hydrogen
deficient. However, the absorption features of C^12C^13 474.4 and C^13C^13 475.2 nm, which
are usually not detectable in spectra of hydrogen deficient carbon stars (Bidelman 1953;
Warner 1967), are seen on our spectra. Even if it is difficult to see them on the tracings
(Fig. 1), they are visible on the original plates. This phenomenon may be related to the
overabundance of s-process elements of this object (Langer et al. 1974).


T. IIJIMA
Astronomical Observatory of Padua,   F. STRAFELLA
Asiago section,                      Department of Physics,
Osservatorio Astrofisico             University of Lecce, Via Arnesano,
I-36012 Asiago (VI) Italy            I-73100 Lecce, Italy



References:

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