Context :

Aims : Given their photospheric origin and refractive nature , SiS molecules can provide important constraints on the relative roles of dust condensation and non-equilibrium processes in regulating the chemistry in circumstellar envelopes around evolved stars .

Methods : New SiS multi-transition ( sub- ) millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented .
A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules .
A detailed radiative transfer modelling of the observed SiS line emission is performed , including the effect of thermal dust grains in the excitation analysis .

Results : We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical models .
The carbon stars ( C/O > 1 ) have a mean fractional abundance of 3.1 \times 10 ^ { -6 } , about an order of magnitude higher than found for the M-type AGB stars ( C/O < 1 ) where the mean value is 2.7 \times 10 ^ { -7 } .
These numbers are in reasonable agreement with photospheric LTE chemical models .
SiS appears to behave similar to SiO in terms of photodissociation in the outer part of the circumstellar envelope .
In contrast to previous results for the related molecule SiO , there is no strong correlation of the fractional abundance with density in the CSE , as would be the case if freeze-out onto dust grains were important .
However , possible time-variability of the line emission in the lower J transitions and the sensitivity of the line emission to abundance gradients in the inner part of the CSE may mask a correlation with the density of the wind .
There are indications that the SiS fractional abundance could be significantly higher closer to the star which , at least in the case of M-type AGB stars , would require non-equilibrium chemical processes .

Conclusions :