Is there any evidence from really old documents?
Wikipedia says "[Charlemagne's] third known concubine was Amaltrud of Vienne. By her he had:
* Alpaida (b.794)"
But Charles's early biographer Einhard did not mention her.
Wikipedia says Charles's son Louis had an illegitimate child Alpais.
The Quebec genealogy site gives her to Louis, with a birth year 793.