Angular diameter measurements of cool giant stars in strong TiO bands and in the continuum

We present angular diameter measurements of 15 giant and supergiant stars in the strong TiO absorption band at 712 nm and in a continuum band at 754 nm with the MkIII long-baseline interferometer. While relatively warm giant stars (spectral types K5 III or M0 III) have identical diameters measured with the two filters, cooler stars (spectral types M3 III to M5 III) appear about 10 percent larger at 712 nm than at 754 nm. This effect is even more pronounced in luminosity class I stars. It can be understood by noting that the tenuous and extended stellar atmospheres of cool stars become opaque in TiO absorption bands at a significantly larger radius than in the continuum; i.e., we measure directly the physical extension of the stellar atmospheres. This is a novel method for testing models of cool star atmospheres.