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planetary and proto - planetary nebulae ( pne , ppne ) present conspicuous departures from spherical symmetry , including e.g. multiple lobes and jets . to explain their evolution from spherical agb envelopes , several models have postulated the presence of dense rings or disks close to the central post - agb stars as the agents of the mechanical collimation of the stellar wind . existing observations reveal the presence of central disks in several ppne , but their limited spatial resolution can not unveil the very inner regions of the disks that are relevant for the processes mentioned above . our first vlba observations of sio masers in oh231.8 + 4.2 , carried out at 7  mm ( @xmath0=2 , @xmath1=10 ) in april 2000 , revealed for the first time the structure and kinematics of the close stellar environment in a ppn ( sanchez contreras et al . , 2002 ) . the sio maser emission arises in several compact , bright spots forming a structure elongated in the direction perpendicular to the symmetry axis of the nebula . such a distribution is consistent with an equatorial torus with a radius of @xmath26  au around the central star . a complex velocity gradient was found along the torus , which suggests rotation and infall of material towards the star . the rotation and infalling velocities deduced are of the same order and range between @xmath27 and @xmath210kms@xmath3 . such a distribution is remarkably different from that found in other late - type stars , where the masers form a roughly spherical ring - like chain of spots resulting from tangential maser amplification in a thin , spherical shell ( diamond et al . , 1994 , desmurs et al . , 2000 ) . the new vlba observations at 3  mm were observed on april 2002 . the data were recorded in dual circular polarization over a bandwidth of 16  mhz , assuming a rest frequency of 86243.442 mhz for the @xmath0=1 @xmath1=21 sio maser emission , for a final spectral resolution of about 0.1 km s@xmath3 . we frequently observed several calibrators for band - pass and phase calibration ; pointing was checked and corrected every less than half an hour . the data reduction followed the standard scheme for spectral line calibration data in aips . to improve the phase calibration solutions , we extensively map all the calibrators , and their brightness distribution were introduced and taken into account in the calibration process . our vlba maps at 3  mm ( fig . 1 ) show two sio maser spots that are globally aligned with those at 7  mm , i.e. , they are roughly distributed orthogonally to the nebular symmetry axis . the velocities of the two maser spots observed at 3  mm are in good agreement with velocities observed at 7  mm . the spatial distribution and velocity structure of the sio maser spots at 3  mm and 7  mm are consistent with a torus with radius @xmath46a.u@xmath5 , perpendicular to the nebular symmetry axis ( inclined @xmath6 with respect to the plane of the sky ) . this torus - like structure is not totally traced by the maser spots . such a spot distribution is expected , even for a homogeneous torus , if the maser is tangentially amplified . this amplification mechanism leads to the most intense maser features at the edges of the torus - like structure . moreover , given the comparable size of the star and the maser emitting region , occultation by the star of the far side of the torus is very likely , which would partially explain the small number of detected spots .    in our opinion , the structure traced by the sio maser emission in oh231.8 + 4.2 , compatible with a rotating+infalling torus very close to the central star , is unlikely an accretion disk . the dynamic is well reproduced by a simple model considering the presence of an inner torus in rotation and infall with a radius of @xmath2  5au ( r@xmath74.5  au ) , which means very close to the surface of the star ( sanchez contreras et al . , 2002 ) . in fact , the dramatic changes with time of the sio @xmath0=2 , @xmath1=10 profile suggest that the masers lie in a region with an unstable structure and/or kinematics , rather than in a stable accretion disk . moreover , accretion disks are expected around the compact companion in a binary system and less likely around the mass - losing star . finally , it is worth mentioning that : ( a ) 3  mm maser spots are located further away from the central star than those at 7  mm , similarly to what has been recently found in agb stars ( see soria - ruiz et al . , these proceedings ) ; and ( b ) the rotation velocity measured in the torus of oh231.8 + 4.2 is consistent with the values of the rotation velocity found in inner envelopes of several agbs ( nml cyg : boboltz & marvel , 2000 ; tx cam and irc+10011 : desmurs et al . , 20001 , r aqr : hollis et al . , 2001 ) . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ boboltz , d. a. & marvel , k. b. , 2000 , , 545 , 149 . + desmurs , j.  f. , bujarrabal , v. , colomer , f. , & alcolea , j.2000 , , 360 , 189 . + desmursj.f . , v. bujarrabal , f. colomer & j. alcolea , 2001 , iau symposium 206 , eds . v. migenes et al . , + hollis , j. m. , boboltz , d. a. , pedelty , j. a. , white , s. m. & forster , j. r. 2001 , , 559 , l37-l40 . + sanchez contreras , c. , desmurs , j. f. , bujarrabal , v. , colomer , f. , alcolea , j. , 2002 , a&a 385 , l1-l4 . + soria - ruiz r. , colomer f. , alcolea j. , desmurs j.- f. , bujarrabal v. , marvel k.b . , diamond p.j . , & boboltz d. , 2003 , these proceedings . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
we present milliarsecond - resolution maps of the sio maser emission @xmath0=1 j=21 ( 3  mm ) in the bipolar post - agb nebula oh231.8 + 4.2 , and compare them with our previous observations of the @xmath0=2 j=10 line ( 7  mm ) . 
 our observations show that the sio masers arise in several bright spots forming a structure elongated in a direction perpendicular to the symmetry axis of the nebula . 
 this , and the complex velocity gradient observed , is consistent with the presence of an equatorial torus in rotation and with an infall of material towards the star . 
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