Planck Collaboration, Ade, P. A. R., Aghanim, N., Armitage-Caplan, C., Arnaud, M., Ashdown, M., et al. (2014). Planck 2013 results - XXV. Searches for cosmic strings and other topological defects. Astronomy & Astrophysics, 571: A25. doi:10.1051/0004-6361/201321621.