A revised classification system in the red for S stars

A classification scheme in the red spectral region for the S stars is described. For those stars exhibiting both ZrO and TiO bands, a modified version of the Keenan (1954) system is used for the assignment of temperature subtypes. For the pure S stars, a new system is introduced utilizing ZrO band strengths and the sodium D lines. Comparisons between the revised types and photometric colors have been made. In addition a new abundance index is proposed based on the relative strength of YO compared to ZrO and TiO. The revised system provides a connecting link between the M and C systems so that all red giants may be classified consistently from the strengths of TiO, ZrO and Na D. (Auth.)