In 1944, Kathleen Kennedy, sister of the future US President, married the heir to which dukedom?

Kathleen Agnes Cavendish, Marchioness of Hartington (née Kennedy; February 20, 1920 – May 13, 1948), also known as "Kick" Kennedy, was an American socialite. She was the daughter of Joseph P. Kennedy, Sr. and Rose Kennedy, sister of future U.S. President John F. Kennedy, and wife of the Marquess of Hartington, heir apparent to the 10th Duke of Devonshire.

When her father was appointed United States Ambassador to the United Kingdom, Kathleen made many friends in London, and was the "debutante of 1938." Working with the Red Cross, she began a romantic relationship with Lord Hartington, whom she married in May 1944. He was killed on active service in Belgium only four months later. Kathleen died in a plane crash in 1948, flying to the south of France on holiday with her married partner the 8th Earl Fitzwilliam.