We present K -band commissioning observations of the Mira star prototype o Cet obtained at the ESO Very Large Telescope Interferometer ( VLTI ) with the VINCI instrument and two siderostats .
The observations were carried out between 2001 October and December , in 2002 January and December , and in 2003 January .
Rosseland angular radii are derived from the measured visibilities by fitting theoretical visibility functions obtained from center-to-limb intensity variations ( CLVs ) of Mira star models ( ) .
Using the derived Rosseland angular radii and the SEDs reconstructed from available photometric and spectrophotometric data , we find effective temperatures ranging from T _ { eff } = 3192 \pm 200 K at phase \Phi = 0.13 to 2918 \pm 183 K at \Phi = 0.26 .
Comparison of these Rosseland radii , effective temperatures , and the shape of the observed visibility functions with model predictions suggests that o Cet is a fundamental mode pulsator .
Furthermore , we investigated the variation of visibility function and diameter with phase .
The Rosseland angular diameter of o Cet increased from 28.9 \pm 0.3 mas ( corresponding to a Rosseland radius of 332 \pm 38 ~ { } R _ { \odot } for a distance of D = 107 \pm 12 ~ { } { pc } ) at \Phi = 0.13 to 34.9 \pm 0.4 mas ( 402 \pm 46 ~ { } R _ { \odot } ) at \Phi = 0.4 .
The error of the Rosseland linear radius almost entirely results from the error of the parallax , since the error of the angular diameter is only approximately 1 % .