The husband of Kathleen Kennedy was heir apparent to the 10th Duke of where?
William Cavendish, Marquess of Hartington Major William John Robert "Billy" Cavendish, Marquess of Hartington (10 December 1917 – 9 September 1944) was an English politician and soldier. He was the elder son of Edward Cavendish, 10th Duke of Devonshire, and his wife, Lady Mary Gascoyne-Cecil. He was the husband of Kathleen Kennedy, sister of the future U.S. President John F. Kennedy.
Earl of Castlemaine The Earldom of Castlemaine was a title created in the Peerage of Ireland. It was created for Roger Palmer, the husband of Barbara Palmer (née Villiers), mistress to King Charles II. The Earl was also given the title "Baron Limerick".