Visualisation of the radial velocity determination process for KIC 4931738. (Upper left) One normalised HERMES spectrum (solid black line) is shown around the Si ii line at 6347 Å with a synthetic binary spectrum (red solid curve) constructed using the determined radial velocities. (Upper right) The χ²-space resulting from a full 2D grid search of the radial velocities of the components. The lowest χ² value is marked with a red filled circle while the accepted value from fitting the χ² values in both dimensions across the best fit value is marked by a white filled circle (almost fully covering the underlying red one). These fits are shown in the (lower panels) with a red line, while the level is plotted with a dashed line. RV_i is expressed in km   s^-1.