The R Coronae Borealis stars - A few mere facts

Abstract

This review presents a selection of recent highlights of observations of R Coronae Borealis (RCB) variables. Emphasis is placed on an abundance analysis of a complete sample (18 stars) of the warm galactic RCBs. It is shown that 14 of the 18 have very similar compositions: the iron mass fraction ranges about a factor of 3 around the solar value (assuming C/He=3%) but abundance ratios X/Fe for elements from Na to Ba show little variation. By contrast, the other 4 stars are deficient in iron but not in Na, Si, S and some other elements. With for example, (Si/Fe) approximately equal to 2, the quartet is indeed 'peculiar'. One of the quartet, V854 Cen shows depletions of elements (other than CNO) similar to the depletions seen in interstellar medium corresponding to average log n (Htot)=1.5. Scenarios for creating RCB from normal single and double stars are summarized.