Some time after Blanche of Lancaster's death in 1368 (most likely in 1371–1372), Swynford and John of Gaunt embarked upon a love affair that would produce four children out of wedlock, but who were subsequently legitimised by the Pope at the time of their parents' marriage; the adulterous relationship endured until 1381, when it was truncated out of political necessity and ruined Swynford's reputation. On 13 January 1396, two years after the death of the Duke's second wife, Infanta Constance of Castile, Swynford and John of Gaunt were married at Lincoln Cathedral. Records of their marriage kept in the Tower and elsewhere list: 'John of Ghaunt, Duke of Lancaster, married Katharine daughter of Guyon King of Armes in the time of K. Edward the 3, and Geffrey Chaucer her sister'. On John of Gaunt's death, Swynford became known as dowager Duchess of Lancaster. She outlived him by four years, dying on 10 May 1403 in her early fifties.