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The fundamental properties of RCBs may be deduced from their spectra
at maximum light. A spectrum represents the overall
distribution of flux with energy (or wavelength). The properties of a
stellar atmosphere may be measured from both the large-scale features
- the broad-band flux distribution - and the detailed spectrum of
absorption lines. Properties of interest are the effective temperature
and the gravity at the stellar surface (these also provide a measure
of the luminosity to mass ratio) and the chemical composition. The
former can be compared with models for stellar structure and
evolution, whilst the composition may provide clues to the past
evolution of the star.
RCB effective temperatures fall mostly around 7000K, although a
small number are as cool as 5500K ([Kilkenny & Whittet 1984],[Asplund et al. 1996]). The
surface gravities are also low, confirming that RCBs have high 
ratios, as suspected from the LMC observations.
The spectra of most RCBs have now been analyzed in detail ([Asplund et al. 2000]).
They show that, in general, 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. They
also show mild enhancements of s-process elements; these are elements
formed as a result of certain nuclei being bombarded by neutrons while
the star was a red giant.
The broad-band flux distributions of RCBs show an excess amount of flux at low energies. This `infrared excess' corresponds to cool material with a temperature of a few hundred degrees Kelvin which surrounds the star ([Kilkenny & Whittet 1984], [Walker 1985]).