Louis XV Pastel Portrait of a Lady in the manner of Jean-Baptiste Perronneau

The reverse with a label THE EARL CATHCART . 

Charles, 9th Lord Cathcart was ambassador to St. Petersburg 1768-1772; his wife Jane, sister of the antiquarian Sir William Hamilton, became a close friend to Empress Catherine the Great. Her mother, Lady Jane Hamilton, daughter of the Earl of Abercorn, was Mistress of the Robes to Princess Augusta of Wales and mistress to Frederick Prince of Wales – she died in Paris in 1753, the year of her youngest daughter’s marriage to Charles Cathcart, and this is conceivably a portrait of her. Her grave is in Montmartre, Paris. The Earldom of Cathcart was created in 1814.