Joint VLTI/VLBA observations of Mira stars

We present preliminary results on a recently started project to perform coordinated observations of Mira stars using near-infrared and radio long-baseline interferometry. We concentrate on recent observations of the Mira star S Ori. Observations with the ESO Very Large Telescope Interferometer (VLTI) were performed to measure the near-infrared diameter of the stellar surface. Concurrent VLBA observations of SiO maser emission towards this Mira star were performed to probe the structure and dynamics of the circumstellar atmosphere. Our near-infrared measurements suggest a diameter change consistent with the photometric period. The SiO maser emission appears to be at a distance of $\sim$ 2 stellar radii ($\Theta_{\rm UD} \sim$ 10.7 mas, $\Theta_{\rm SiO} \sim$ 20 mas, both at stellar phase $\sim$ 0.7), and to show a clumpy distribution within a ring-like structure.