  OH231.8+4.2 is an archetypal pre-planetary nebulae (pPN). It is a binary
system surrounded by bipolar nebula. Some years ago the authors extensively
studied it and performed several VLBI observations from which they obtained
mas-resolution maps of the SiO (7 mm) and H2O (1.3 cm) maser emissions. H2O
masers were found to be distributed in two areas along the symmetry axis of the
nebulae oriented nearly north-south delineating a bipolar outflow and their
astrometric positions were accurately measured. SiO masers, indicating the
position of the Mira component of the binary system, form a structure
perpendicular to the axis of the nebulae. The general picture of the source
looked satisfactory, except for the relative position of the two masers.
Surprisingly, SiO masers, were tentatively placed 250 mas away (370 AU) from
the apparent center of the outflow. Using the ALMA we observed the SiO maser
emission at 86 GHz and accurately derived the position of the Mira component.
Combining our previous VLBA data and our new ALMA observations we are now able
to give a more complete and detailed description of the inner part of this
amazing pPN.
