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.
Bouchard, Constance B. "Childeric III And the Emperors Drogo Magnus and Pippin the Pious." Medieval Prosopography, Vol 28(2013) https://www.jstor.org/stable/44946900 Page 4, footnote 10 clarifies eighth century sources refer to Alpaidis and Plectrude both as wives. She is also referred to in the body of the article as Pippin's "bigamous second wife."(Also page 4).