Abstract
This paper will be concerned with those variables lying on the cool side of the cepheid strip in the region of the HR diagram occupied by the red giant and supergiant stars. The blue edge of the red variable region for old disk population stars has been found by Eggen (1973b) to be at (R-I)K˜ 0.9 (log Teff˜ 3.53). According to Eggen (1973a, 1973b), all giant stars cooler than this vary, although the amplitude is very small (ΔV~0.2 mag) near the blue edge. In the halo population the blue edge is considerably hotter at (R-I)K˜ 0.5 (log Teff˜ 3.62). On the other hand, some small amplitude red variables (eg. R Dor, AK Hya, IQ Her) have been found at the low temperatures (RK-RK > 1.6) where large amplitude red (Mira) variables exist. From these results, it appears that the position of the blue edge deoends on some unknown stellar parameter(s). Auman (1971) has suggested that the helium abundance may be involved.
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