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We present infrared interferometric imaging of the S-type Mira star ChiCygni. The object was observed at four different epochs in 2005-2006 with theIOTA optical interferometer (H band). Images show up to 40% variation in thestellar diameter, as well as significant changes in the limb darkening andstellar inhomogeneities. Model fitting gave precise time-dependent values ofthe stellar diameter, and reveals presence and displacement of a warm molecularlayer. The star radius, corrected for limb darkening, has a mean value of 12.1mas and shows a 5.1mas amplitude pulsation. Minimum diameter was observed atphase 0.94+/-0.01. Maximum temperature was observed several days later at phase1.02+/-0.02. We also show that combining the angular acceleration of themolecular layer with CO (Delta v = 3) radial velocity measurements yields a5.9+/-1.5 mas parallax. The constant acceleration of the CO molecules -- during80% of the pulsation cycle -- lead us to argument for a free-falling layer. Theacceleration is compatible with a gravitational field produced by a2.1(+1.5/-0.7) solar mass star. This last value is in agreement withfundamental mode pulsator models. We foresee increased development oftechniques consisting in combining radial velocity with interferometric angularmeasurements, ultimately allowing total mapping of the speed, density, andposition of the diverse species in pulsation driven atmospheres.

The Pulsation of Chi Cygni Imaged by Optical Interferometry; a Novel Technique to Derive Distance and Mass of Mira Stars