Upper plot: response of the highly magnetically sensitive FeH Q₃(2.5) line at 9975.476 Å and the marginally sensitive FeH R₁(2.5) and R₁ (6.5) lines at 9976.397 Å and 9978.721 Å, respectively, to changes of 200 K in the effective temperatures for the range of 2600–3400 K. The line profiles become deeper with cooler temperatures. A magnetic field strength of 3000 G was applied and instrumental (0.3 Å) broadening. Lower plot: same for changes of log g from 4.0 to 5.0 in steps of 0.5 and an effective temperature of 3000 K.