Normalised excess variance versus log(L_{2−10   keV}) for the ULXs. Arrows indicate the 90% confidence upper limits on for the sources with negative excess variance estimates. The point plotted with an empty square indicates the M 82 X−1 measurement when corrected for the contribution of X42.3+59 to the observed count rate (see text for details). The shift of M 82 X−1 is shown as a dotted line. The solid line indicates the best fit to the data (excluding NGC 55 ULX).