The king died in 1685. In March 1691, eighteen-year-old Barbara gave birth to an illegitimate son of the Earl of Arran, whom she named Charles Hamilton (1691-1754). Arran's parents bitterly opposed his relationship with Barbara. Right after giving birth, she became a nun in the English Priory of St. Nicholas, at Pontoise in Normandy, France, taking the name Sister Benedicta, where she later became prioress in 1721. Her son Charles was raised by her mother, the Duchess of Cleveland, who supposedly disowned her.
An autograph of the Prioress is as follows:
"Mon nom du monde est Barbe Fitz Roy est en Religion Benedite fille Du Roy De La grande Bretagne Charles 2dc j'ay fait profession dans Le Couuent des Benedictines Angloiscs De Pontoise L'annee 1691 Le 2* D'auril c'est maison est mittige."
Roughly translated as:
"My name in the world is Barbe Fitz Roy, in Religion it is Benedicta, daughter of the King of Great Britain, Charles II. I made profession at the Convent of the English Benedictines in Pontoise, the year 1691, the 2nd of April. It is my place of penance."
The Lady Barbara died there in monastery on 6 May 1737, and lies buried in the church of the Priory.