  The mass loss process along the AGB phase is crucial for the formation of
circumstellar envelopes (CSEs), which in the post-AGB phase will evolve into
planetary nebulae (PNe). There are still important issues that need to be
further explored in this field; in particular, the formation of axially
symmetric PNe from spherical CSEs. To address the problem, we have conducted
high S/N IRAM 30m observations of 12CO J=1-0 and J=2-1, and 13CO J=1-0 in a
volume-limited unbiased sample of semi-regular variables (SRs). We also
conducted Yebes 40m SiO J=1-0 observations in 1/2 of the sample in order to
complement our 12CO observations. We report a moderate correlation between mass
loss rates and the 12CO(1-0)-to-12CO(2-1) intensity ratio, introducing a
possible new method for determining mass loss rates of SRs with short analysis
time. We also find that for several stars the SiO profiles are very similar to
the 12CO profiles, a totally unexpected result unless these are non-standard
envelopes.
