Illustration of the cross-correlation method. Top: N V 4604, 4620 Å doublet that is used for cross-correlation, shown for SMC AB9. The co-added spectrum as well as the spectrum that was taken on observing night 0 are displayed. For reference, we also show spectrum 0 shifted by 40 km/s. The spectra are normalized and then vertically shifted such that the integrated flux in the shown wavelength range is zero. Bottom: CCFs of the N V 4604, 4620 Å doublet for the co-added template spectrum and the six individual spectra that were taken. For this figure, the spectra were not corrected for barycentric motion.