  On this paper, we present simultaneous VLBA observations of the J=1-0 and
J=2-1 rotational lines in the v=1 and v=2 vibrationally excited states of SiO
(at 7 and 3 mm wavelengths). We have mapped these four maser lines in the
circumstellar envelopes of three AGB stars: IRC +10011, Chi Cyg and TX Cam. We
study the relative spatial distribution between these maser lines. In
particular, for the two v=1 transitions, we found that the J=1-0 and J=2-1
maser spot distributions are unalike, challenging all the current theoretical
pumping mechanisms for SiO masers (which predict quite similar distribution for
maser lines in the same vibrational excited state).
