These are among the strangest of all variable stars, and also peculiar in that their normal (i.e., non-variable) state is maximum, so while the dwarf novae have outbursts, these have fades. This is still a rather mixed bag of stars, and the class appears to include some stars which do not conform to the classical R Coronae pattern.
They are typically yellow stars, not too unlike the Sun, but with a very high proportion of Carbon in their atmospheres. At sufficiently low temperatures this carbon will condense out into vast clouds of soot, and cut off large amounts of light from the star, though a perusal of some of the following will show you that astronomers are by no means as agreed as this! If these clouds are in our line of sight, we will see the star fade, sometimes by us much as eight magnitudes in the case of R Coronae or RY Sagittarii. Naturally, these variations are completely random and unpredictable, both in style and duration, though typically the recovery to maximum is slower than the fall, and tends to be interrupted by irregular fluctuations.
Many of these stars, RY Sgr being the best example, show independent, quasi-periodic fluctuations obviously of some pulsatory character, and they appear to be related to the Cepheid variables in this respect, though those stars do not contain the abnormal proportion of carbon in their atmospheres as do the RCB stars.
The following interesting snippets are from the AAVSO's "Variable star of the Month" and includes links to several other useful astrophysical sites.
The variable star R Coronae Borealis is a jewel worthy of a place in any crown. It is one of the most interesting and most peculiar of all variables, and is often called the 'ideal' irregular variable. Its times of minima are distributed absolutely at random, according to the laws of pure chance" (Margaret Mayall, The Review of Popular Astronomy, May/June 1962, p33). R Coronae Borealis (R CrB) has been a favorite with observers ever since its discovery nearly 200 years ago by the English amateur, Edward Pigott. Located inside the bright circlet of stars that form the Northern Crown, R CrB is usually easy to find with binoculars or even the unaided eye at 6th magnitude. The critical contribution that amateurs have made to astronomy by observing R CrB is articulated on the ISO (Infrared Space Observatory)/ESA (European Space Agency) 12 June, 1996 information note page and is quoted below. The ISO homepage may be located at http://www.iso.vilspa.esa.es/ The ESA homepage may be found at http://www.esa.int/
"Shortly before ISO's launch, amateur astronomers reported that the star called R Coronae Borealis was fading from view. This elderly star is normally quite easy to see with binoculars, but intermittently it puffs off clouds of dust that almost hide it from view. Professional astronomers do not have the time to monitor irregularly variable stars, and rely on amateurs to alert them to such events like that in R Coronae Borealis. A few months later when the star could be seen only with powerful telescopes, ISO obtained an infrared spectrum of the star in just one minute, using the high-speed spectroscopic facility of the photometer ISOPHOT. "We caught this star smoking," says Helen Walker of the Rutherford Appleton Laboratory in England, who was in charge of the observation. "The amateurs saw the star fade from view in visible light in October, but it remained bright in the infrared. The telltale wavelengths revealed sooty carbon compounds newly formed in the star's vicinity. Without ISO [and amateur astronomers] we could not hope to analyse such a striking event."
R CrB is currently recovering from a period of fadings that started in August of 1999. The AAVSO encourages observers to monitor this elusive star and report their observations to headquarters. For more information on joining, or submitting observations, visit their site.
R Coronae Borealis is the prototype star of the R Coronae Borealis (RCB) type variables. These hydrogen-deficient and carbon-rich F or G supergiants go into outburst not by brightening like other variables, but by fading! R CrB spends most of its time at maximum around magnitude 6, and at irregular intervals it experiences deep declines of up to 8 magnitudes. The decline is sharp. It may drop several magnitudes in a few weeks or even days. The star may remain faint for an extended period of time or have several recoveries and declines in succession. Often the final rise back to maximum light is slow, taking several months to a year. Examples of light-curves of some of these stars appear farther down the page.
R Coronae Borealis is an unusual star worthy of much attention, observation, and research. This abstract from Geoffrey C. Clayton's Invited Review Paper, "The R Coronae Borealis Stars", Publications of the Astronomical Society of the Pacific, 108, 1996 March, gives us a clue why these stars are so interesting:
"This year  marks the bicentennial of the discovery of the variability of R Coronae Borealis. The R Coronae Borealis (RCB) stars are distinguished from other hydrogen-deficient objects by their spectacular dust-formation episodes. They may decline by up to 8 magnitudes in a few weeks, revealing a rich emission-line spectrum. Their atmospheres have unusual abundances with very little hydrogen and an overabundance of carbon and nitrogen. The RCB stars are thought to be the product of a final helium shell flash or the coalescence of a binary white-dwarf system. Dust may form in non-equilibrium conditions created behind shocks caused by pulsations in the atmospheres of these stars. The RCB stars are interesting and important, first because they represent a rare, or short-lived stage of stellar evolution, and second because these stars regularly produce large amounts of dust so they are laboratories for the study of dust formation and evolution."
R Coronae Borealis stars stay at maximum and then intermittently experience fluctuating minima because carbon-rich dust clouds periodically obscure the photosphere of the star. R Coronae Borealis, when exposed, is a star that usually shines around 6th magnitude, and it is during this time that the star is at maximum. At highly irregular time intervals that are unpredictable as of yet, the star enters a deep minimum. The minimum is caused by a dust cloud of amorphous carbon (extinction curves have confirmed that the dust causing the decline is carbon-rich), which eclipses the photosphere of R CrB, preventing an observer from seeing the entirety of the starís luminosity. Each successive drop in brightness within a decline is caused by new dust formation. These episodes of dust formation seem to occur on successive pulsational cycles of the star, although this is hard to prove. There are different explanations of the formation and evolution of the dust clouds and it is not quite predictable yet when, why, or how they form. Eventually, the dust cloud moves out of the way, re-exposing the photosphere of the star as it returns to maximum.
One theory that explains the formation of dust clouds around RCB type stars has been called the Orbiting Dust Cloud Theory. This model, as explained in Geoffrey C. Clayton's Invited Review Paper "The R Coronae Borealis Stars", PASP, 108, 1996 March, proposes that clouds of dust orbit the RCB star and periodically pass along the line of sight of the star, obscuring its photosphere. This model, however, has trouble fitting the observational data. The passage of a dust cloud across the star is not consistent with the structure of the decline light curve or with evidence of dust-grain evolution. So dust ejection from a secondary star in a binary system must be invoked to make this model viable, however, there is no evidence of binarity in RCB stars.
A second, more likely model explained in the same article involves dust forming from material lost from the RCB star itself. Mass is lost from the star and then moves away from it until it reaches the condensation temperature of carbon dust (at about 20 stellar radii). Once the carbon dust has formed, the photosphere of the star is eclipsed, the star goes into minimum, and emission lines appear. When the dust is blown away by radiation pressure the photosphere of the star may again be seen. This model, dubbed the Dust Puff Theory, is a good one except that it implies a physical connection between the stellar atmosphere and the location of the dust formation (which is hard to imagine for dust forming at 20 stellar radii). One way to avoid this problem is to say that the dust forms closer to the star and is blown away by radiation pressure. But then one might expect and rightly so, assuming thermodynamic equilibrium, that the region very near the star is too hot to enable the condensation of carbon into dust.
Even now, after over two centuries of observation, many aspects of the RCB phenomenon remain mysterious, including the details of the dust formation mechanism, the evolutionary status of RCB stars, and the nature of their emission-line regions.
SU Tau is probably the most active of all these stars, and currently (24 Feb, 2000) is still below visibility for most visual observers, though at maximum it is an easy object of the tenth magnitude, possibly even visible with good binoculars on the border with Orion. The AAVSO (f) chart for this star goes down to magnitude 17, which as you can see from the lightcurve below, is necessary! Why some of these stars should be so active while others are not is a mystery, though it may be possible that some stars which have been called R Coronae stars are not in fact members of the group.
Whilst I can't claim this is one of my favourite RCB stars (since it is too far south for me to see) it is certainly one of the most interesting. One reason why several R Coronae stars remain undiscovered is simply because the active ones spend a lot of time in their faint state - and this was the case with V854 Cen. At maximum it is an easy binocular object that shows continual, sinusoidal variation by about half a magnitude, as do several of these stars. It is included in my Handbook of Binocular Astronomy. Just click on Centaurus to see the other goodies this Southern constellation has to offer. If, like me, you can't see Centaurus from where you live, then try the following Northern stars.
Some RCB stars are active, some are more leisurely. This is definitely one of the latter variety; but all the more reason to watch it, of course. The nature of irregular variables is that they spring surprises on us, and though I have been looking at this eleventh-magnitude star for several years now without seeing anything very drastic other than fairly small variations of about half a magnitude, I still take a peek whenever I can. You never know - that time you decide not to look could be the start of a dramatic piece of action. Again, I speak from experience!
Another inactive star of magnitude 11, but one which lies in a beautiful field adjoining a wide double of the 7th magnitude. It lies not far from the well-known Mira variable V Cas and you could well take a look at them once a week - though only actually make estimates of V Cas once a fortnight. This is quite a red star, which may account for the large scatter in estimates in the lightcurve below, even though UV may well have been doing the "sinusoidal variation" thing as well.
This is a rather more active star of the same normal brightness as the two preceding objects, and can be found close to a star of magnitude 8, which is a degree North of a wide, bright double near the border with Aquila. It has been known to fade to the fifteenth magnitude, and is as I write this (June 2005) going through a fade. Like all this class, you need to look at this object on every clear night, though if nothing untoward is happening, weekly estimates are probably OK.
For a long time, up until about 10 years ago in fact, this star was thought to be a Long-Period variable (goodness only knows why!) and observations by amateurs were very scanty. At maximum, it is of magnitude 11, and since it lies very close to two other stars of similar brightness and is one of the most Northerly variables in the whole sky, it is a wonder that its true nature went unsuspected for so long. Add to that the fact that it lies close to the well-known star R Camelopardalis and is one of the more active R Coronae stars, and here you have an ideal object for small telescopes. It can be a bit difficult to find on the first few occasions, but you'll soon get the hang of it!
Below appears the AAVSO provisional light-curve for this star over about the past two years, which shows several deep fades. This is a star that needs more, and continuous, observing. So - why are you reading this? Go on, get to it!! I've even supplied a link to the AAVSO so you can download a chart...