According to Wikipedia, Byzantium Emperor Constantine XI Palaiologos and The Empress Helena, acting as regent, appealed to Ottoman Sultan Murad II.
Just wondering why they appealed to Ottoman Sultan's consulting ?
Despite the foreign and domestic difficulties during his reign, which culminated in the fall of Constantinople and of the Byzantine Empire, contemporary sources generally speak respectfully of the Emperor Constantine. When his brother, Emperor John VIII Palaiologos, died childless, a dispute erupted between Constantine and his brother Demetrios Palaiologos over the throne. Demetrios drew support by opposing the union of the Orthodox and Catholic churches. The Empress Helena, acting as regent, supported Constantine. They appealed to the Ottoman Sultan Murad II to arbitrate the disagreement.