Theuws, Frans (2001). "Maastricht as a centre of power". In Frans Theuws (ed.). Topographies of Power in the Early Middle Ages. Mayke B. de Jong, Carine van Rhijn. BRILL. pp. 190–91. ISBN9789004117341. Retrieved 13 October 2015.
McDougall, Sara."Chapter Two: The Carolingian Example: The Sons of Concubines." Royal Bastards: The Birth of Illegitimacy, 800-1230. First Edition. Oxford, Oxford University Press, 2016. Page 82. ISBN978-0-19-182763-1doi:10.1093/acprof:oso/9780198785828.001.0001, March 2024. McDougall makes it clear that the modern-day concepts of 'concubines' or 'second wives' are not the same as in the Medieval era.
Mcdougall, Sara."Chapter One:The Language of Illegitimacy in Medieval Europe." Royal Bastards: The Language of Illegitimacy, 800-1230Oxford, 2016. ISBN978-0-19-182763-1doi:10.1093/acprof:oso/9780198785828.001.0001 Pages 49 and 53. Page 53 is specifically referring to and expanding upon Brigitte Kasten's research.