http://en.wikipedia.org/wiki/Katherine_Swynford
Katherine Swynford, Duchess of Lancaster (also spelled Catherine[2] and Synford), née (de) Roet (also spelled (de) Rouet, (de) Roët, or (de) Roelt) (probably 25 November 1350 – 10 May 1403), was the daughter of Sir Payne (or Paen/Pain/Paon) (de) Roet (also spelled (de) Rouet, (de) Roët or (de) Roelt), originally a Flemish herald from County of Hainaut, later knighted.
Katherine became the third wife of John of Gaunt, Duke of Lancaster, a son of King Edward III. Their descendants were members of the Beaufort family, which played a major role in the Wars of the Roses. Henry VII, who became King of England
in 1485, derived his claim to the throne from his mother Lady Margaret Beaufort, who was a great-granddaughter of Gaunt and Katherine Swynford.