Frequency of fading events

The most obvious and spectacular properties of RCB stars are their dramatic fading events. They spend the majority of their time at maximum light. Initial decline is sudden and steep, the star can fade by up to 8 magnitudes in a few weeks. The decline may show a series of standstills, partial recoveries and subsequent declines. The final recovery to maximum light may take several months to a year. This aspect of RCB behaviour represents an area where amateur astronomers consistently make a major contribution - particularly by alerting the research community to the onset of a fading event. Finding charts and up-to-the-minute light curves are provided online by, for example, the American Association of Variable Star Observers (http://www.aavso.org/), or the Variable Star section of the British Astronomical Association Variable Star Section (http://www.telf-ast.demon.co.uk/).

The first outline of a mechanism for the fading events was proposed in the 1930's when it was ``shown that the shape of the light curve of R CrB and its spectral variations at minimum can be accounted for by supposing it to eject matter which condenses at a considerable distance and forms obscuring clouds. The solid matter is believed to be principally carbon'' ([O'Keefe 1939]).

From nearly two centuries of observations of RCrB, a characteristic interval of 1100 days between fading events has been deduced ([Feast 1986]). Several searches have consistently failed to find any regular patterns in RCB declines which might help to identify a causal factor such as rotation or binarity (e.g. [Sterne 1935], [Howarth 1977], 1978, [Dick & Walker 1991], [Clayton et al. 1993]). For two cases only, RYSgr and V854Cen, it has been shown that the onset of the decline episodes may be closely correlated with a particular phase of the pulsation cycle ([Pugach 1977], [Lawson et al. 1992]). It is attractive to relate the onset of a dust formation episode to the pulsation of the stellar envelope, but the evidence for such a connection is not yet compelling.

The ratio between the characteristic interval between fades, and the typical pulsation period of RCBs implies that a decline occurs about once every twenty pulsation cycles ([Feast 1986]).

**Figure 2:** RCB dust-cloud ejection and evolution (based on a figure by [Clayton 1996]). If a dust cloud is ejected in the line of sight it initially obscures the star, then becomes more transparent as it expands. The outer cool dust shell is regulalrly replenished by these ejected clouds. It re-radiates stellar flux in the infrared, so that even when the star is obscured in the line of sight, the shell continues to reflect the pulsations at the stellar surface.
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