R Coronae Borealis variable
An R Coronae Borealis variable ( abbreviated RCB , R CrB ) is an eruptive variable star that varies in luminosity in two modes , one low amplitude pulsation ( a few tenths of a magnitude ) , and one irregular , unpredictably-sudden fading by 1 to 9 magnitudes . 
<triplet> eruptive variable <subj> star <obj> subclass of