We present infrared interferometric imaging of the S-type Mira star \chi Cygni .
The object was observed at four different epochs in 2005-2006 with the IOTA optical interferometer ( H band ) .
Images show up to 40 \% variation in the stellar diameter , as well as significant changes in the limb darkening and stellar inhomogeneities .
Model fitting gave precise time-dependent values of the stellar diameter , and reveals presence and displacement of a warm molecular layer .
The star radius , corrected for limb darkening , has a mean value of 12.1 mas and shows a 5.1 mas amplitude pulsation .
Minimum diameter was observed at phase 0.94 \pm 0.01 .
Maximum temperature was observed several days later at phase 1.02 \pm 0.02 .
We also show that combining the angular acceleration of the molecular layer with CO ( \Delta v = 3 ) radial velocity measurements yields a 5.9 \pm 1.5 mas parallax .
The constant acceleration of the CO molecules – during 80 % of the pulsation cycle – lead us to argument for a free-falling layer .
The acceleration is compatible with a gravitational field produced by a 2.1 ^ { +1.5 } _ { -0.7 } solar mass star .
This last value is in agreement with fundamental mode pulsator models .
We foresee increased development of techniques consisting in combining radial velocity with interferometric angular measurements , ultimately allowing total mapping of the speed , density , and position of the diverse species in pulsation driven atmospheres .