It is very difficult to prove with absolute certainly an exact line leading from the Merovingians to anyone. Although the Merovingians were most certainly related to each other, the exact nature of the relationships is open to some speculation. A person generally believed to be a daugher or son could just as easily be a niece or nephew.

     However, it appears quite certain that Bertha "Broadfoot", wife of King Pépin and mother of Charlemagne was the granddaughter of a Merovingian princess who was perhaps a daughter of Thierry (Theuderic) III who was in turn a grandson of Dagobert I (d. 639).  We have the following from George Andrews Moriarty: The Plantagenet Ancestry of King Edward III and Queen Philippa, page 232:
 

The Counts of Laon      M. L. Levillain, one of the most careful and able of the modern French scholars, in a paper entitled "Les Nibelungen Historiquies et leurs Alliances de Famille" in Annanles du Midi (vol. 49, (1937), pp. 337 - 408) and vol. 50 (1938), pp. 5 - 66) by employing nomenclature, the charters and chronicles has succeeded in establishing, amond other families, a very probable and convincing pedigree of the Counts of Laon from whom are descended the mother of Charlemagne and also, most probably the family of St. William Count of Auture and Marquis of Septimania and undoubtedly the later Counts of Toulouse. Bertha, a Merovingian princess (husband unknown) Founded Prüm Abbey. Perhaps dau. of Thierry III, King of Austrasia, Neustria and Burgundy (d. 691) Caribert (Herbert) Count of Laon 720 - 747 Bertha "au grand pied" m. King Pepin "le bref" d. 12 July 783                        d. 768 The Carolingians

     The above passage was transcribed from page 232 of G. A. Moriarty, The Plantagenet Ancestry of King Edward III and Queen Philippa, Mormon Pioneer Genealogical Society, Salt Lake City, 1985 and an image of this is shown next. This book seems to be very rare and almost unobtainable. Professor Moriarty never published his work, apparently because he felt it was never "finished". After his death (circa 1960?) his manuscript was donated to the New England Historic Genealogical Society in Boston. In 1985, The Mormon Pioneer Genealogical Society produced a photographic reproduction of the original manuscript in Professor Moriarty's own hand.