Plots of the HCN and SiO integrated intensity upper limits (K km s﹩^{-1}﹩) vs. CO (1–0) integrated intensity toward BM Gem compared with the measurements in oxygen‐rich and carbon‐rich envelopes reported by Bujarrabal et al. (1994). The solid square shows the upper limit for the broad plateau, while the open square shows the upper limit for the narrow peak. In each panel, the solid line shows the trend for carbon‐rich stars (solid circles) while the dashed line shows the trend for oxygen‐rich stars (open circles).