Discovery observations of the Zeeman signature in σ Lup at the AAT on December 28, 2007, (top) and four days later (bottom), at magnetic phases 0.14 and 0.47, respectively. Each figure shows the LSD Stokes unpolarised I (lower panel) and circularly polarised V (top panel) profiles of SEMPOL spectra, all normalised by the continuum. The middle panel displays the (null) N profile for integrity purposes. The integrated Zeeman signature in the V profile over the width of the line (between the outer vertical dashed lines) gives the value of the longitudinal component of the magnetic field, integrated over the stellar surface. On the first night we obtained B_l = 79 ± 20 G, whereas on the second night the V signature changed sign, and we obtained B_l =  − 69 ± 27 G. Note the irregular bumps in the I profiles, which indicate the presence of spots on the stellar surface.