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 (Bessel et al. 1996, Hofmann et al. 1998, Tej et al. 2003). 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 +/- 200 K at phase phi=0.13 to 2918 +/- 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 +/- 0.3 mas (corresponding to a Rosseland radius of 332 +/- 38 R_sun for a distance of D=107 +/- 12 pc) at phi=0.13 to 34.9 +/- 0.4 mas (402 +/- 46 R_sun) 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 %.
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