CO J=2-1, HCN J=1-0, HNC
J=1-0, SiS J=5-4, HC
N J=10-9, CN N=1-0, CS J=5-4, SiO
J=3-2, SO 
=6
-5
, and 
CO J=2-1 (observations
from the literature, including results for the lines 
CO
J=1-0, CS J=3-2 and SiO J=2-1, are also taken into account).
We confirm the presence of two emitting shells in S Sct and TT Cyg.
We also find a two-shell structure in U Cam and argue that the envelope of
R Scl detected in molecular emission is also probably detached. The
(very probable) detection of HCN emission from the inner envelope of S Sct
and its unexpected two-peak structure are remarkable. In the inner
shells of S Sct and U Cam and the outer shells of U Cam and R Scl, the
chemistry is found to be dominantly C-rich, from the comparison of the line
intensity ratios with those usually found in standard evolved stars. We
note the lack of detected molecular emission, other than CO lines,
from the outer envelopes around S Sct and TT Cyg, for which we cannot
conclude about the dominant chemistry. We have then not found any sign of
O-rich chemistry in the observed detached shells around C-rich evolved
stars.