[18] Much of the time it shows variations of around a tenth of a magnitude with poorly defined periods that have been reported as 40 and 51 days.

These correspond to the first overtone and fundamental radial pulsation modes for an extreme helium star slightly under one M☉.

Typically the star starts to return to maximum brightness almost immediately from its minimum, although occasionally this is interrupted by another fade.

The sudden drop in brightness may be caused by a rapid condensation of carbon-rich dust similar to soot, resulting in much of the star's light being blocked.

[19] R Coronae Borealis at maximum light shows the spectrum a late F or early G yellow supergiant, but with marked peculiarities.

The normal absorption spectrum is replaced by emission lines, especially HeI, CaII, NaI, and other metals.

At minimum, the spectrum shows the development of carbon clouds that obscure the photosphere, leaving chromospheric lines visible at times.

The absolute magnitude of −5 is calculated by comparison with R CrB variables in the Large Magellanic Cloud whose distances are known quite accurately.

[22] The Gaia data release 1 parallax also gives a distance of 1.4 kpc although with a considerable margin of error.

[23] 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.

[22] Evolutionary models of post-AGB stars give a mass of 0.66 M☉ for R CrB, but with a considerable margin of error.

[25] The obscuration appears to happen closer to the star as clouds of carbon condense at shock regions in an expanding front.