AGB stars are divided into three different spectral types depending on their photospheric C/O-ratio; M-type with C/O<1, carbon stars with C/O>1, and the S-type stars believed to have a C/O-ratio close to 1. This is also believed to be an evolutionary sequence (M, S, C), as more and more carbon is dredged-up from deeper nuclear-burning layers within the star. By studying the mass-loss rate and chemical characteristics of the different types, a deeper understanding of the mass-loss rate mechanism(s) and the chemical evolution along the AGB may be achieved.
During the last couple of years, we have gathered a unique data-base of circumstellar molecular radio line emission observations toward a sample of 40 S-type AGB stars. I will present the results from the modelling of the abundances of SiO and HCN of these stars. The results will also be compared to that previously found for M-type stars and carbon stars.
