Sketch of the effect of a magnetic spot on RV measurements. Top: rotating star with large spot, covering 5% of the visible surface if observed at disk center, seen at three phases. The star is rotating with the spot approaching in the left column, centered in the middle column, and rotating out of view in the right column. Second row: stellar line profile broadened by rotation but without spot flux, showing the impact of a dark star spot. Third row: flux emerging from spot only, assuming the same temperature as in the photosphere. Row four: sum of surrounding and the spot region. Row five: radial velocity determined from fitting a Gaussian profile to the spectral lines in row four. In rows three to five, black solid lines show the case without a magnetic field (and zero spot temperature contrast). Blue and red dashed lines show cases in which the spot area has B = 1 kG and 2 kG, respectively (λ = 1.2   μm and g = 1.0).