  We have performed VLBA observations of the SiO v=1 and v=2 J=1-0 masers in
two AGB stars, TX Cam and IRC +10011. We confirm the ring-like spatial
distribution, previously found in several AGB objects, as well as the
tangential polarization pattern, already reported for TX Cam. Both properties,
that seem to be systematic in this kind of objects, are characteristic of
radiatively pumped SiO masers. On the contrary, we do not confirm the previous
report on the spatial coincidence between the J=1-0 v=1 and 2 masers, a result
that would have argued in favor of collisional pumping. We find that both lines
sometimes arise from nearby spots, typically separated by 1-2 mas, but are
rarely coincident. The discrepancy with previous results is explained by the
very high spatial resolution of our observations, ~ 0.5 mas, an order of
magnitude better than in the relevant previously published experiment.
