 Complete the following phrase:  Konstantin married, but his spouses name is unknown, and from this marriage Konstantin had a daughter, Jelena, who in 1392 married Byzantine Emperor Manuel II Palaiologos r. 1391-1425. Manuel II and Jelena had several children, among whom were the last Byzantine Emperors John VIII r. 1425-1448 and Constantine XI r. 1449-1453. Constantine XI, the last Byzantine Emperor, who died 