Top panel: r-band polarisation curve of 4U 0614+091. With black squares we reported the polarisation detection () with their error bars, whereas with red arrows we represented all the upper limits that we evaluated when a polarisation measure was not possible, due for example to lower S/Ns. The superimposed horizontal line represents the value P_mean obtained by fitting the polarisation detections and upper limits with a constant. The gray band represents the 1σ level. Note that a better determination of the polarisation level is obtained from the probability density function (as discussed in the text), resulting in a similar central value but a much smaller error (0.96%). Bottom panel: r-band polarisation curve of one field star chosen as reference (black squares: polarisation detections; red arrows: upper limits).