A photoelectric index of MgH + Mg b for late-type stars has been created by the addition of one filter bandpass to the DDO system. Measurements show that this index has good sensitivity to surface gravity for K stars, and suggest that it can differentiate metal-poor halo giants from disk stars. From this index, involving measurement through two filters alone, it appears that membership can be determined for stars on the giant branch of globular clusters. It is possible that the index could be used, after calibration with cluster giants, to determine ages of giant stars in the field.
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Clark et al. (1979) studied this question.