The spectroscopic activity and the short term variability in peridic variable Be stars appear correlated. Balona et al. (1992) studied photometrically Eri for several years, and found the maximum amplitude of the periodic light variation in coincidence with the increased spectroscopic activity reported by Smith (1989). Cuypers et al. (1989) observed Cen in an unusual high state of photometric activity three months after an H outburst reported by Ghosh at al. (1987). Conversely, our data (see Sect. 2) show a coincidence between a phase of constant brightness and the lack of any Be activity, as indicated by the absence of emission in the lines. The general picture that emerges from these facts is that the photometric variability is related to the active status of the Be star, dissapearing during the pure absorption-line phases.
Rapid aperiodic variability, both spectroscopic and photometric, has been reported for several Be stars. Peters (1986) observed the development of an H emission line in Centauri in only two days. Gorrod et al. (1993) detected a significant diminution in the H line strength of X Persei over a period of thirty minutes. Sudden photometric brightenings on 1-2 days timescales have been observed in CMa (Balona 1993) and Cap (Balona 1990). Fading events, with an amplitude of several tenths of mag. and lasting a few days have been already observed in CrB (Roark 1971) (see Sect. 1), and in the Be/X-ray binary LS I +61 303 (Fabregat, unpublished). Such fast and aperiodic variability is usually interpreted in terms of local mass ejection episodes from the surfaces of Be stars (Smith 1994).
The fading event we have observed is of much lower amplitude and shorter time scale than those reported above. As far as we know episodes of similar characteristics are not described in the literature. Probably the main reason is the difficulty of detection. The observational evidence seems to indicate than episodic short time scale variability is a common feature of Be stars. However its unpredictable nature and sometimes low amplitude make it extremely difficult to detect. Extended observing campaigns and accurate photometry would be needed to find other similar events. It is important to monitor stars such as CrB to be able to get a better idea of duration of its active and quiescent phases.
© European Southern Observatory (ESO) 1998
Online publication: December 8, 1997