Multiline Zeeman signatures through line addition

Context. To obtain a significant Zeeman signature in the polarised spectra of a magnetic star, we usually add the contributions of numerous spectral lines; the ultimate goal is to recover the spectropolarimetric prints of the magnetic field in these line additions. Aims. Here we want to clarify the meaning of these techniques of line addition; in particular, we try to interpret the meaning of the pseudo-line formed during this process and to find out why and how its Zeeman signature is still meaningful. Methods. We create a synthetic case of line addition and apply well tested standard solar methods routinely used in research on magnetism in the Sun. Results. The results are convincing and the Zeeman signatures well detected; Solar methods are found to be quite efficient for stellar observations. We statistically compare line addition with least-squares deconvolution and demonstrate that they both give very similar results, as a consequence of the special statistical properties of the weights. Conclusions. The Zeeman signatures are unequivocally detected in this multiline approach. We suggest that magnetic field detection is reliable well beyond the weak-field approximation. Linear polarisation in the spectra of solar type stars can be detected when the spectral resolution is sufficiently high.