Hyperfine structure (hfs) of the J = 1﹩_{0,1}﹩ ← 0﹩_{0,0}﹩ rotational transition of HDO recorded using the Lamb-dip technique (P = 0.19 mTorr, modulation depth = 8 kHz; in pink). Simulated spectra based on computed hyperfine parameters are shown in order to point out how the hfs changes from the mere consideration of the deuterium nucleus (in blue; without SR: dashed line), by the inclusion of hydrogen (in dark blue), and by the further incorporation of the SS interaction constants (in violet). The consideration of the crossover resonances (marked by asterisks) leads to the final predicted spectrum (in purple).