  OH 231.8+4.2 is a well studied preplanetary nebula (pPN) around a binary
stellar system that shows a remarkable bipolar outflow. To study the structure
and kinematics of the inner 10-80 AU nebular regions we performed
high-resolution observations of the H$_2$O 6$_{1,6}$--5$_{2,3}$ and $^{28}$SiO
$v$=2, $J$=1--0 maser emissions with the Very Long Baseline Array. The absolute
position of both emission distributions were recovered using the phase
referencing technique, and accurately registered in HST optical images. H$_2$O
maser clumps are found to be distributed in two areas of 20 mas in size
spatially displaced by $\sim$60 milli-arcseconds along an axis oriented nearly
north-south. SiO masers are tentatively found to be placed between the two
H$_2$O maser emitting regions, probably indicating the position of the Mira
component of the system. The SiO maser emission traces an inner equatorial
component with a diameter of 12 AU, probably a disk rotating around the M-type
star. Outwards, we detect in the H$_2$O data a pair of polar caps, separated by
80 AU. We believe that the inner regions of the nebula probably have been
altered by the presence of the companion, leading to an equator-to-pole density
contrast that may explain the lack of H$_2$O masers and strong SiO maser
emission in the denser, equatorial regions.
