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The Acta Sanctorum is an official collection of saints' lives by various authors used for their processes of beatification or canonization.
St. Dympna's vita is pp. 475-495 of Acta Sanctorum vol. 16; p. 476 says the author is the canon regular Peter of St. Aubert of Cambrai (Cameracensis):
Vitam S. Dympnæ scripsit Petrus Canonicus Regularis S. Autberti Cameracensis
Molanus, Miræus, the Roman Martyrology, Henschenius, t. 3, Maij. p. 477