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We report on K-band VLTI/AMBER observations at medium spectral resolution($\sim$1500) of RS Capricorni, an M6/M7III semi-regular AGB star. From thespectrally-dispersed visibilities, we measure the star diameter as a functionof observing wavelength from 2.13 to 2.47 microns. We derive a Rosselandangular diameter of $7.95 \pm 0.07$ mas, which corresponds to an effectivetemperature of $3160 \pm 160$ K. We detect size variations of around 10% in theCO band heads, indicating strong opacity effects of CO in the stellarphotosphere. We also detect a linear increase of the size as a function ofwavelength, beginning at 2.29 microns. Models of the stellar atmosphere, basedon the mass of the star as estimated from stellar-evolution models, predictCO-size effects about half of those observed, and cannot reproduce the linearsize increase with wavelength, redward of 2.29 microns. We are able to modelthis linear size increase with the addition of an extended water-vapor envelopearound the star. However, we are not able to fit the data in the CO bandheads.Either the mass of the star is overestimated by the stellar-evolution modelsand/or there is an additional extended CO envelope in the outer part of theatmosphere. In any case, neither the water-vapor envelope, nor the CO envelope,can be explained using the current models.

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