LSD profiles of χ Cyg (in stellar rest frame velocity) from Narval observations collected in March 2012 (average of 174 V sequences). Bottom: Stokes I (the unpolarized spectrum). Middle: null profile (extended by a factor of 1000) and its associated ±1σ error bar (on the left side). Top: Stokes V profile (extended by a factor of 1000) and its associated ±1σ error bar (on the left side). In all panels, the vertical dotted lines help to appreciate the location and contribution of the blue line component of the Stokes I profile. The two straight lines reported around the Stokes I profile help to delimit the velocity domain used for the determining of B_// (see Sect. 5.1).