  The S-stars have been suggested to be a brief transitional phase as stars
evolve from oxygen-rich M-type stars into carbon stars, through the dredge up
of carbon from He-shell burning. As possible transition objects, S-stars might
help achieve a deeper understanding of the chemical evolution as a star ascends
the AGB, as well as shed more light on the mass-loss mechanism. We have
initiated a large survey of 40 S-stars to observe line emission in common
molecules such as CO, SiO, HCN, CS and SiS. Detailed radiative transfer
modelling of multi-transition CO radio line observations towards a sample of 40
S-stars shows that the mass-loss rate distribution of S-stars is consistent
with those found for M-type AGB stars and carbon stars. Initial results from
modelling of the circumstellar SiO emission are also presented.
