She became a nun in the Benedictine house at Dunkirk, France. Catherine FitzCharles, born in 1658, was the illegitimate daughter of Charles II of England and his mistress, Catherine Pegge. Her older brother by one year, Charles FitzCharles, was made the 1st Earl of Plymouth by his father. Little is known about Catherine's life, but she is thought to have become a Benedictine nun at Dunkirk Abbey in Dunkirk, France, like many other highborn Englishwomen during the reign of Charles II. She is believed to have resided there under the religious name, Sister Ophelia, until her death in 1759 at the impressive age of 101. However, the shockingly small amount of information available about Catherine FitzCharles leads some to conclude that Catherine died in infancy or early childhood instead.
"Charles quietly slept with his new mistress, Catherine Pegge. By her he had an illegitimate daughter who died and a son who was somewhat unimaginatively named Charles FitzCharles". 
"Seven mistresses had provided him with twelve bastards. Lady Castlemaine was the mother of five of them, Nell Gwynn of two, while Elizabeth Killigrew, Catherine Pegge, Moll Davis and Louise de Kerouaille had added one each to the number.
Mother of two children, Charles and Catherine. 
Thepeerage only gives two children. 
Daughter Catherine died in infancy. 
Alison Weir give three children, Charles 1657-1680, Katherine 1658-1759, a nun at Dunkirk, and an un-named daughter possibly another Katherine, conjectural. Said to have died young. Material is unsourced. 
"Catherine Pegge the beautiful daughter of a Derbyshire Squire presented him with a son in 1657 and a daughter in 1658. The girl died". 
"As far as one can keep track of them, the total that survived long enough to feature in the royal records was a round dozen. The twelve know bastards were born from seven women; Lucy Walter, Elizabeth Killigrew, Catherine Pegge, Barbara villiers (five), Nell Gwynn (two), Moll Davis and Louise de Kerouaille." 
Catherine in not mentioned in the long list of names at Who were the Nuns? A Prosopographical study of the English Convents in exile 1600-1800: Dunkirk. 
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