A revised spectral classification system in the red for S stars.

Low-dispersion observations of S stars in the region 5450-7000 A have been used to establish a revised temperature classification scheme for these objects. Bands of TiO and ZrO and the Na D lines are found to be useful in placing all S type stars on a common temperature scale. Temperature subtypes for those objects exhibiting both ZrO and TiO bands are assigned by a modified version of the Keenan 1954 system. For the pure S stars, a new system is introduced utilizing the ZrO bands and the D lines. Comparisons between the revised types and photometric colors demonstrate an improvement over Kennan's system, especially for the pure S stars. Further, a new abundance index is proposed based on the relative strength of the bands of YO as compared to ZrO and TiO. It appears that this index is fundamentally related to the C/O ratio, though it may also be affected somewhat by the general enhancement of s-process elements.