Left: Hα+[N ii] narrow-band image of the V842 Cen extended shell on a logarithmic grey scale. Right: comparison of the radial profile of V842 Cen in the Hα+[N ii] narrow-band image after subtracting the average standard star profile (dashed line) with cross sections averaged perpendicular to the dispersion (±5 Å around the Hα emission line centre) in the long-slit spectrum before (dot-dashed line, Fig. 7 left) and after (continuous line, Fig. 7 right) removing the stellar continuum and the Hα emission line contamination. The profile and the cross sections were normalized to their maxima.