  We have undertaken toward 30 Mira or semi-regular variables and one OH/IR
object highly sensitive observations of the v=1, J=2-1 and 3-2 transitions of
SiO simultaneously with observations of the J=1-0 transition of CO during three
observing sessions in the period 1995 to 1996. As in our previous observations
of 1994, we observe that for several stars the SiO profiles exhibit unusually
broad wings which sometimes exceed the terminal velocity of the expanding
circumstellar envelope traced by the thermal CO emission. We have discovered a
clear dependence of the SiO wing emission on the optical phase. These wings are
probably due to peculiar gas motions and varying physical conditions in
relation with the stellar pulsation. However, we cannot exclude other
mechanisms contributing to the observed wings. In particular, SiO turbulent
motions for the semi-regular variables or the asymmetric mass loss mechanism
may play a role. We conclude that the SiO wing emission is due to masing
processes and that this emission very likely arises from the inner part of the
circumstellar envelope.
