  In this paper, we report on 606 S-type stars identified from Data Release 9
of the LAMOST medium-resolution spectroscopic (MRS) survey, and 539 of them are
reported for the first time. The discovery of these stars is a three-step
process, i.e., selecting with the ZrO band indices greater than 0.25, excluding
non-S-type stars with the iterative Support Vector Machine method, and finally
retaining stars with absolute bolometric magnitude larger than -7.1. The 606
stars are consistent with the distribution of known S-type stars in the
color-magnitude diagram. We estimated the C/Os using the [C/Fe] and [O/Fe]
provided by APOGEE and the MARCS model for S-type stars, respectively, and the
results of the two methods show that C/Os of all stars are larger than 0.5.
Both the locations on the color-magnitude diagram and C/Os further verify the
nature of our S-type sample. Investigating the effect of TiO and atmospheric
parameters on ZrO with the sample, we found that log g has a more significant
impact on ZrO than Teff and [Fe/H], and both TiO and log g may negatively
correlate with ZrO. According to the criterion of Tian et al. (2020), a total
of 238 binary candidates were found by the zero-point-calibrated radial
velocities from the officially released catalog of LAMOST MRS and the catalog
of Zhang et al. (2021). A catalog of these 606 S-type stars is available from
the following link https://doi.org/10.12149/101097.
