a carbon-enhanced extreme helium star. Modelling the pulsations suggests that the star's mass is . The temperature at maximum is reasonably well known at 6,900K and appears to decrease during the fades as the photosphere is obscured by condensing dust. The distance of R Coronae Borealis is not known exactly, but is estimated at 1.4 kiloparsecs from assumptions about its intrinsic brightness. The absolute magnitude of −5 is calculated by comparison with R CrB variables in the Large Magellanic Cloud whose distances are known quite accurately. The luminosity is estimated from helium star models to be and the star has a radius around . The Gaia data release 1 parallax also gives a distance of 1.4 kpc although with a considerable margin of error. There is a fainter star 3" away from R Coronae Borealis, but it is believed to be a distant class K dwarf. Its colour and apparent magnitude are not consistent with being at the same distance as R Coronae Borealis. Formation There are two main models for the formation of R CrB stars: the merger of two white dwarfs; or a very late helium flash in a post-AGB star. Models of post-AGB stars calculate that a star with the appearance of R CrB would have a mass around so it is thought to have formed by the merger of a carbon-oxygen white dwarf and a helium white dwarf. The detection of significant lithium in the atmosphere is not easily explained by the merger model, but is a natural consequence of a late helium flash. Evolutionary models of post-AGB stars give a mass of for R CrB, but with a considerable margin of error. Circumstellar material Direct imaging with the Hubble Space Telescope shows extensive dust clouds out to a radius of around 2000 astronomical units from R Coronae Borealis, corresponding to a stream of fine dust (composed of grains about 5 nm in diameter) associated with the star's stellar wind, and coarser dust (composed of grains with a diameter of around 0.14 µm) ejected periodically. The obscuration appears to happen closer to the star as clouds of carbon condense at shock regions in an expanding front. "Puffs" of dust emitted from the star condense at about from the surface, and are visible as cometary knots when they lie to side of the star. There is also a shell about 4 pc wide containing dust at 25