  We have observed the bipolar post-AGB candidate OH 231.8+4.2, using the
mid-infrared interferometer MIDI and the infrared camera with the adaptive
optics system NACO on the Very Large Telescope. An unresolved core (<200 mas in
FWHM) is found at the center of the OH 231.8+4.2 in the 3.8 micron image. This
compact source is resolved with the interferometer. We used two 8-meter
telescopes with four different baselines, which cover projected baseline
lengths from 62 to 47 meters, and projected position angles from 112 to 131
degrees that are almost perpendicular to the bipolar outflow. Fringes from 8 to
9 micron and from 12 to 13.5 micron were clearly detected, whilst the strong
silicate self-absorption allows only marginal detection of visibilities between
9 and 12 micron. The fringes from the four baselines consistently show the
presence of a compact circumstellar object with an inner radius of 30-40 mas,
which is equivalent to 40-50 AU at 1.3 kpc. This clearly shows that the
mid-infrared compact source is not the central star (3 AU) but circumstellar
material. The measured size of the circumstellar material is consistent with
the size of such disks calculated by hydrodynamic models, implying the
circumstellar material may have a disk configuration.
