The fundamental properties of RCBs may be deduced from their spectra at maximum light. A typical spectrum looks like that of an F or G supergiant. RCB effective temperatures fall mostly around 7000K, although a small number are as cool as 5500K. The spectra of most RCBs have now been analyzed in detail. They show that hydrogen makes up less than one part per thousand down to less than one part per million of the STELLAR ATMOSPHERE. Most of the atmosphere is neutral helium. Although carbon is enriched, it is not possible to establish its abundance reliably. All RCB atmospheres show products of hydrogen- and helium-burning NUCLEOSYNTHESIS in several different episodes, as well as mild enhancements of s-process elements.