R Coronae Borealis star

R Coronae Borealis star A type of eruptive variable that undergoes occasional fades of variable amplitude, with a sudden onset and generally slower recovery, often with major fluctuations; abbr. RCB. The fades have depths of 1–9 mag., and last from months to years. The onset, depth, and duration of the fades are unpredictable. They occur when clouds of carbon particles in the outermost atmospheric layers are ejected in the direction of the observer, and absorb the light from the underlying photosphere. When the photosphere is obscured the star enters a chromospheric phase in which the residual light originates in the deep overlying chromosphere. The stars are high-luminosity supergiants of spectral types B–M and R, hydrogen-deficient but rich in helium and carbon. The majority also appear to be pulsating variables with amplitudes of up to a few tenths of a magnitude, and periods of 30–100 days. Although few in number (approximately two dozen are known), R Coronae Borealis stars are significant for studies of stellar evolution. It is now thought that they arise from the merger of two white dwarfs. The prototype, R Coronae Borealis itself, is of spectral type F or G and varies between mags. 5.7 and 14.8.