Degradation of the S/N in the spectrum of a star with a nearby neighbour, as a function of the distance to the neighbouring source (normalized to the width of the PSF) and of the magnitude difference (star minus neighbour) between the two stars. We define the relative S/N as the ratio between the measured value and the value expected for an isolated star. For clarity, small horizontal offsets have been applied to the plotted datapoints for the different magnitude differences.