Brightness ratio around the minor disk axis in the Q_φ frames for V 721 CrA and BN CrA (the latter after a Gaussian smoothing with a 0.5 pixel sigma). The mirror axis inclination is orthogonal to the best PA values reported in Sect. 3.1 (namely, 104.4^° for V 721 CrA and 117.5^° for BN CrA, east-of-north to the disk major axis). The coronagraph area is covered by a grey circle. North is up and east to the left.