Thankyou Justin; and I also re-found the Henry Project link http://sbaldw.home.mindspring.com/hproject/prov/alpai000.htm that you'd pointed me in the direction of before.
Settipani discusses the debate about the paternity of Alpais, preferring the theory that Emperor Charles I was her father[69]. If Emperor Louis was her father, it is unlikely that she was born before [793/94], given his known birth date in 778. It would therefore be chronologically tight for her to have had [three] children by her husband before his death in 816.
But actually not really because it makes her 23yrs old when he dies. Charlemagne’s wife Hildegard had had 9 children by the time she died at 23 or 24yrs.