I don't know what authority Weis cites as I do not have access to his work.
Someone else will have answer your direct question. I am familiar with the
primary source material and the main work done on this material. The
maternity of Pepin is inferred and hence is not completely certain. The
basic evidence is a list in the liber confaternitatum augiensis* of Charles
Martel's family. Relationships are not stated but the groupings are
deliberate (as is the case in such lists). From other sources we know most
of the relationships, and in this list The males are listed in order and
then their spouses. The sequence of the first four female names are
significant here - Ruadtrud, Ruadheid, Svanahild regina, Bertha regina.
Bertha regina is the wife of Pippin rex. Pippin rex immediately follows
Karolus major domus in the list. We also know from other sources that
Svanahild regina was the wife of Karolus m.d. whom he married (or whatever)
in 725. 'Ruadtrud' is a variant of 'Chrotrud'. Three sets of annals state
that in 725 that Chrotrud died or was dead. This fits with the date of
Charles Martel's union with Swanahild. Also the name 'Chrotrud' occurs
amongst Charles' and Pepin's immediate known descendants. Ruadheid also
seems to have had a union with Charles at the same time as he had a union
with Chrotrud. Indeed there has been an extensive discussion in the
literature whether the early Carolingians went in for polygamy or some form
of official concubinage. The strong transmission of the name 'Chrotrud'
amongst Pepin's descendants leads to the highly probable inference that
Ruadtrud/Chrotud was his mother.
There is a discussion of these unions in Settipani's *La prehistoire des
Capetiens 481-987: Premiere Partie*, 167-173.
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