We performed the first VLBI observations of the SiO v =1 and v =2 J =1–0 masers in a Proto Planetary Nebula OH 231.8+4.2 ( also known as the Rotten Egg nebula ) , at milliarcsecond resolution .
Only the v =2 maser transition was detected .
We detect several maser spots lying along a line which is almost perpendicular to the axis of symmetry of the Nebula .
We find that all the emission is concentrated in an area of \sim 10 mas , indicating that the SiO masers are originated very close to the surface of the star .
One of the two emission areas presents an elongated structure with a clear velocity gradient .
The detected emission is consistent with a torus , or disk , in rotation with a velocity of \sim 6 km/s and with an infall velocity of \sim 10 km/s .