AMBER observations of the AGB star RS Capricorni: extended atmosphere and comparison with stellar models

We report on K-band VLTI/AMBER observations at medium spectral resolution (similar to 1500) of RS Capricorni, an M6/M7III semi-regular AGB star. From the spectrally dispersed visibilities, we measure the star diameter as a function of observing wavelength from 2.13 to 2.47 microns. We derive a Rosseland angular diameter of 7.95 +/- 0.07 mas, which corresponds to an effective temperature of 3160 +/- 160 K. We detect size variations of around 10% in the CO band heads, which are indicative of strong opacity effects of CO in the stellar photosphere. We also detect a linear increase in the size as a function of wavelength, beginning at 2.29 microns. Models of the stellar atmosphere, based on the mass of the star estimated from stellar-evolution models, predict CO-size effects of about half the sizes observed, and cannot reproduce the linear size increase with wavelength, redward of 2.29 microns. We are able to model this linear size increase with the addition of an extended water-vapor envelope around the star. However, we are unable to fit the data in the CO bandheads. Either the mass of the star is overestimated by the stellar-evolution models and/or there is an additional extended CO envelope in the outer part of the atmosphere. In any case, neither the water-vapor envelope nor the CO envelope can be explained using the current models.