Measurements of the aspect ratio δ and angular width a_w of the CME void in STEREO-A HI1 images, plotted against heliocentric distance. Panel a: observed δ (black dots), mean and standard deviation (red solid and dashed lines), and a blue line at δ = 1 indicates the value for a circular cross–section. Panel b: angular width a_w derived from STEREO-A HI1 images (black dots), with mean and standard deviation as solid and dashed red lines.