Centre-to-limb variation of LSD line profiles (a) and continuum intensity (b). Panel (a) compares Stokes IVQ LSD profiles computed with full radiative transfer calculations (black symbols connected with solid lines) to those obtained using the single-line UR approximation for b = 5 (thick red lines) and b = 0.27 (thin blue lines). The latter value is consistent with the continuum limb-darkening coefficient η = 0.21. In all cases the magnetic field strength is 1 kG and the field vector is inclined by 45° relative to the line of sight. Panel (b) compares the continuum limb-darkening predicted by detailed radiative-transfer calculations (black symbols connected with solid lines) with a linear limb-darkening law using η = 0.21 (red line).