S-type stars are defined as cool giants which show evidence of bands of ZrO in their spectra. V Cnc, a semi-regular variable star, shows pronounced bands of ZrO, whereas o1 Ori is a marginal S-type star, as it shows only marginal evidence of ZrO (see the ZrO bandhead at 5551, barely visible in o1 Ori). V Cnc shows almost no evidence for TiO in its spectrum, whereas the bands of TiO in o1 Ori are as strong as those in an M4 giant. The strong hydrogen emission lines in the spectrum of V Cnc are due to shock waves in its pulsating atmosphere. Some S-type stars also show molecular bands due to YO, VO and LaO. The chemical peculiarities in S-type stars are also thought to arise from the dredge-up of nuclear processed material from deep within the star.
This figure uses 3.6 Å resolution flux spectra.