We present the first coordinated VLBA/VLTI measurements of the stellar diameter and circumstellar atmosphere of a Mira variable star .
Observations of the v = 1 ,J = 1 - 0 ( 43.1 GHz ) and v = 2 ,J = 1 - 0 ( 42.8 GHz ) SiO maser emission toward the Mira variable S Ori were conducted using the VLBA .
Coordinated near-infrared K -band measurements of the stellar diameter were performed using VLTI/VINCI closely spaced in time to the VLBA observations .
Analysis of the SiO maser data recorded at a visual variability phase 0.73 show the average distance of the masers from the center of the distribution to be 9.4 mas for the v = 1 masers and 8.8 mas for the v = 2 masers .
The velocity structure of the SiO masers appears to be random with no significant indication of global expansion/infall or rotation .
The determined near-infrared , K -band , uniform disk ( UD ) diameters decreased from \sim 10.5 mas at phase 0.80 to \sim 10.2 mas at phase 0.95 .
For the epoch of our VLBA measurements , an extrapolated UD diameter of \Theta _ { \mathrm { UD } } ^ { K } = 10.8 \pm 0.3 mas was obtained , corresponding to a linear radius of R _ { \mathrm { UD } } ^ { K } = 2.3 \pm 0.5 AU or R _ { \mathrm { UD } } ^ { K } = 490 \pm 115 ~ { } R _ { \odot } .
Our coordinated VLBA/VLTI measurements show that the masers lie relatively close to the stellar photosphere at a distance of \sim 2 photospheric radii , consistent with model estimates .
This result is virtually free of the usual uncertainty inherent in comparing observations of variable stars widely separated in time and stellar phase .