Although, in a historical view (see Section 1.1), R Coronae Borealis stars – usually addressed as R CrB stars – were the first kind of objects observed exhibiting pronounced occultations that seemed to appear episodically in their light curves (Figure 18.1). Despite the considerable endeavor in particular in the last century to physically and chemically reveal this conspicuous stellar behavior, there is still no final explanation of these spectacular occultations based on a consistent modeling satisfying all observational and theoretical aspects.

This unsatisfactory situation also concerns the conclusive physical explanation of the occultation process itself, as, in a more general sense, classification of the real evolutionary state of R CrB stars, all exhibiting a dramatic atmospheric hydrogen deficiency (εH,R CrB/εH,⊙ ≤ 10−3) accompanied by an unusual large carbon abundance (εC,R CrB/εC,⊙ ≥ 3 to 10), constitutes characteristic features of this small yet important class of spectacular objects. R CrB stars in total seem to populate an extremely narrow stellar mass range between 0.8 and 0.9 M⊙ yet exhibit a broad distribution of their effective temperature, ranging from 3,500 K, which indicates a spectral type R (e.g., S Aps), up to values around 15,400 K, which fits to spectral type early B (e.g., MV Sgr). In this range, the standard star R CrB with Teff ≃ 7,000 K occupies some intermediate position corresponding to a spectral class F.