These rare stars are technically known as erratically variable luminous supergiants. About 40 stars are known to belong to this class. The most famous is R Coronae Borealis, which has been studied for over 150 years.
R Cor Bor, as it is often called, has yet to show any pattern at all as it disappears and reappears, seemingly at will. The disappearance is thought to result from the sudden coming of a thick cloud of atmospheric soot that dramatically obscures its light. It simply "smokes up". The cause is unknown. The star then plunges as much as eight magnitudes. Its sudden reappearance is equally unpredictable. The soot disappears (probably through re-absorption) and the star shines once again.
It is quite difficult to analyze the light of these stars when they are misbehaving due to their obscuration by the soot. But it is thought that they are senile old stars which have exhausted their hydrogen fuel and are now reduced to burning carbon -- hence the soot.
Other well-known members of this class include RY
Sagittarii, S Apodis, SU Tauri, XX Camelopardalis and RS
Telescopii.
But of just these five stars, three have peculiarities that lend
credence
to their class being known as the oddballs of the F stars. For
example:
RY Sgr shows a
G-type spectrum which is incompatible with its red color; there are
serious reservations about XX
Cam belonging to this class at all; and RS Tel shows a surprising and
unexpected infrared excess
due to a surrounding disk of cold dust. Thus the smokers are
also oddballs.