Helena Jans van der Strom

Wikipedia on Francine Decsartes (Helena's daughter):

Francine was the daughter of Helena Jans van der Strom, a domestic servant of Thomas Sergeant – a bookshop owner and associate of Descartes at whose house in Amsterdam Descartes lodged on 15 October 1634. When Descartes moved back from Amsterdam to Deventer the following winter, Helena went with him. Although Francine was referred to as an illegitimate child, her baptism in Deventer on August 7, 1635, was recorded among the legitimate births. Helena officially remained Descartes' servant, and René referred to Francine as his niece, but both were included in his life. In 1640 Descartes wrote that he would bring his daughter to France to learn the language and be educated, but before that could happen, Francine died of scarlet fever at the age of 5. Russell Shorto postulated that the experience of fatherhood and losing a child formed a turning point in Descartes' work, changing its focus from medicine to a quest for universal answers.

Helena was the only woman with whom Descartes is known to have been intimate and she and Descartes appear to have remained close after Francine's death. Helena may have moved with Descartes to his next addresses – including in 1643 to Egmond-Binnen – where in 1644 she married the local innkeeper Jan Jansz van Wel. Notary acts discovered by Jeroen van de Ven show that Descartes provided the 1000-guilder dowry for this wedding. Descartes himself would remain in Egmond-Binnen until 1649, the longest period he ever stayed at any residence.

http://en.wikipedia.org/wiki/Helena_Jans_van_der_Strom