  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 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 $7.95 \pm 0.07$ mas, which corresponds to an effective
temperature of $3160 \pm 160$ K. We detect size variations of around 10% in the
CO band heads, indicating strong opacity effects of CO in the stellar
photosphere. We also detect a linear increase of the size as a function of
wavelength, beginning at 2.29 microns. Models of the stellar atmosphere, based
on the mass of the star as estimated from stellar-evolution models, predict
CO-size effects about half of those 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 not able 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.
