## 1. IntroductionThe effective temperature of a star characterizes the radiometric properties of the atmosphere in a purely formal but very useful way. Effective temperatures may not in general be determined from either photometric colors or from atomic and molecular excitation, since the effective temperature is a global property and not a local property. Where a computed model atmosphere is available for a star, the effective temperature of the model will be meaningful to the extent that the model contains the required physics and boundary values. A more "direct" determination of the effective temperature, possible with little or no model dependence, may be obtained, according to the definition of effective temperature, from two observables - the apparent bolometric flux and the apparent angular diameter. Steady improvements in techniques of optical interferometry make it possible to conduct increasingly focused studies of stellar effective temperature. Here we report an extension of the calibration of the effective temperatures of M giants to spectral types as late as M8. These will be useful as benchmarks in evaluating model atmospheres for very cool stars, and in many applications, such as correlating model luminosity functions with observed colors for composite systems. © European Southern Observatory (ESO) 1998 Online publication: February 16, 1998 |