Sharon, I don't have a copy of La préhistoire des Capétiens and I don't have photocopies of the relevant pages. (I should probably just buy a copy. I need it often enough, but used copies are usually about $200 and I don't find a copy for sale at Abe Books right now). WorldCat shows me the closest library to me is at University of Kansas, 500 miles away. There is also a copy at NEHGS in Boston. If we get desperate we can appeal to one of the users who are members there to get a copy.
I think Settipani's argument is based on chronology. This seems to be confirmed by a post at SGM. This post presents three reconstructions for the ancestry of Alpais (and daughter Susanna), based on Werner, Hlawitschka, and Settipani. A summary quote from Moriarty says, "She [Alpais] is often called a natural daughter of the Emperor Lewis the Pious but other authorities style her a natural daughter of Charlemagne, which is undoubtedly, for chronological reasons, correct."
https://groups.google.com/forum/?hl=en#!searchin/soc.genealogy.medi...
A snippet view of La préhistoire des Capétiens from Google Books on p 202 says:
... avoir abordé de façon détaillée la question, la plus grandes vraisemblance est un faveur d'une fille de Charlemagne. Flodoard apporte trois renseignements généalogiques que l'on ne retrouve pas dans les annales de Lorsh:
-- l'épouse de Beggo se nommair Alpais
-- elle était fille de l'empereur Louis Ier
-- ses enfants se nommaient Eberhard et Leuthard
La source de son information est une donation que fit Alpais à ses deux fils pour un ...
Rough translation:
... addressed the question in detail, the greater likelihood is for a daughter of Charlemagne. Flodoard gives three pieces of genealogical information not found in the annals of Lorsh:
- The wife is Beggo is named Alpais
- She was the daughter of Emperor Louis I
- Their children were named Eberhard and Leuthard
The source of his information is a donation that Alpais made to her two sons for ...
So, Settipani is arguing with a contemporary source, apparently Flodoard. I thought it might be helpful to follow up on that.
German Wikipedia says Alpais was a daughter of Louis I. I like German Wikipedia for these things because the Germans often take more time to detail the sources than do the English and French.
http://de.wikipedia.org/wiki/Beggo_I.
German Wikipedia cites the Annales Hildesheimenses (815): Biego de amici regis, qui et filiam imperatoris nomine Elpheid duxit uxorem, eo tempore defunctus est. (Beggo, of the king's friends, and who took to wife a daughter of the emperor named Alpais), died at this time (815).
http://www.dmgh.de/de/fs1/object/display/bsb00000870_00051.html?sor...
It also cites Ann. Laur. Min (Cod Fuldensis): "Picco, primus de amicis regis, qui et filiam imperatoris duxit uxorem, defunctus est." [ nomine Elpheid ] Ann. Hildesh. (Beggo, first among the king's friends, and who took to wife a daughter of the emperor, died).
And it provides the citation to Flodoard. Hist. Rémens. eccles. p.448: "Quod cenobium postea per precariam ipsius Alpheidis vel filiorum eius Letardi et Ebrardi ad partem et possessionem Remensis devenit ecclesie."
This just says "Alpais as well as her sons Leuthard and Eberhard", so I wanted to see the the full context from Flodoard.
Flodoard says, ""Quod monasterium Ludowicus Alpheidi, filie sue, uxori Begonis comitis, dono dedit eidemque sacro loco immunitatis praeceptum delegavit, ut et pater ipsius, Karolus imperator, egerat olim. Quod cenobium postea per precariam ipsius Alpheidis vel filiorum eius Letardi et Ebrardi ad partem et possessionem Remensis devenit ecclesie."
http://www.dmgh.de/de/fs1/object/display/bsb00000606_00455.html?con...
So, yes. In Flodoard, Alpais wife of Count Beggo is the daughter of Louis, who is identified as the son of Charlemagne.
I don't know that we can go further than this. Two contemporary sources say Alpais was daughter of an emperor. A third, Flodoard, a few generations later specifically says she was daughter of Louis, but the chronology (apparently) fits better for Charlemagne.
I just we combine the different Alpaises into one (if we haven't) already, make an arbitrary choice about where to put her, cite Flodoard, and detail what we know about the arguments in the About Me. I'd be inclined to follow Flodoard, reasoning that Settipani is just a theory. A very excellent theory maybe, but still just a theory. I wouldn't be upset if someone else would prefer to follow Settipani. Since we don't know absolutely, someone will argue no matter what we choose.