Acknowledgements

We would like to thank Egidio Landi Degl’Innocenti and Marco Landolfi for sharing with us their deep knowledge on the physics of the spectral line polarization, which they have described in great detail in their rigorous monograph on “Polarization in Spectral Lines”. Finantial support by the Spanish Ministry of Education and Science through projects AYA2007-63881 and the European Commission through the SOLAIRE network (MTRN-CT-2006-035484) is gratefully acknowledged.