This is her brief Wikipedia article: "Marie Louise d’Esparbès de Lussan, by marriage vicomtesse then comtesse de Polastron (Bardigues, 19 October 1764 - London, 27 March 1804) was a member of the Esparbès de Lussan family and the mistress of the comte d’Artois, who later reigned as Charles X of France.
Life - The daughter of Louis François d’Esparbès de Lussan and Marie Catherine Julie Rougeot (1746-1764), she was a lady-in-waiting to queen Marie Antoinette. She married Denis de Polastron 1758-1821, half-brother to Yolande de Polastron, countess, then duchess of Polignac, governess of the royal children (known as the Enfants de France) and intimate friend of the queen.
The comte d’Artois was a great lover of beautiful women and first saw Louise at the Versailles court. He became so lastingly attached to her that he made her his 'favorite' in title (though he had no children by her). When she died of tuberculosis in 1804, the comte d’Artois was so deeply attached to her that he decided to swear a vow of perpetual chastity. His private life after her death became blameless, and he also became devoted to religion, often enthusiastically supporting the Ultramontanist movement within France's Roman Catholic Church."