Adrien Baillet La vie de M. Descartes vol.1 Ch.11, p. 146 (year 1626) in google books
M. De Sainte Croix was another distinguished arithmetician, but even
more - an intimate friend of M. Descartes. I believe that he is the same one
that we find called by other people André Jumeau, who was prior of the
Sainte Croix (Holy Cross), and who had been tutor to M. Le Duc De
Verneuïl. M. Descartes testified that he esteemed very particularly
the profound knowledge which M. De Sainte Croix had of arithmetic and
algebra: and he took a singular pleasure in answering his questions,
because he found almost as much satisfaction in them as M. De Sainte
Croix testified to this for his answers. He died before M Descartes.
There is some ambiguity as he is most often named by contemporaries "Monsieur de Sainte Croix". His name appears frequently enough in connection with Descartes and his circle (Mersenne, Fermat, etc.). 1588-1651 are the dates of his life as given by notes to editions of the letters exchanged. He has not published anything, Mersenne notes, but passing remarks are easily found; search has not discovered some encyclopaedical entry about him.