1995A&A...293..198S

47 S stars have been searched for circumstellar CO (J=1-0 and/or 2-1) emission, and 29 have been detected, including 4 which show no evidence of dust in their IRAS LRS spectra, and one with possibly no Tc (and therefore not an AGB star). Six stars show anomalous features in their profiles, showing the presence of more than one kinematic component in the expanding outflow. Two stars may have detached-shell envelopes. The expansion velocity distribution for S stars envelopes is different than that for C-rich stars, with the former having a slightly lower mean expansion velocity, and a significantly higher fraction of objects with very low expansion velocities (≲5.5km/s). In most S stars, the mass-loss rates are >2x10^–7M_☉/yr and the gas-to-dust ratios are >1000. Our detection of CO in S stars with little or no detectable dust implies substantial mass-loss in these objects. The expansion velocities and mass-loss rates of the relatively dust-free stars show a much steeper dependence on the the far-infrared excess ({DELTA}IR_e), as compared to the more dusty stars. This suggests that when the amount of dust becomes small, mass-loss may be partially driven by a different mechanism than radiation pressure on grains, which probably dominates in the dusty envelopes.