Constantine Doukas (co-emperor)

Constantine Doukas
Emperor of the Romans
Cloisonné engraving of Constantine Doukas from the Holy Crown of Hungary
Byzantine emperor
Reign	c. 1074–1078
Predecessor	Michael VII Doukas
Successor	Nikephoros III Botaneiates
Co-emperors	Michael VII (1071–1078) Konstantios (1071–1078) Andronikos (1068–1070s)
2nd reign	1081–1087
Predecessor	Nikephoros III Botaneiates
Successor	Alexios I Komnenos
Born	c. 1074
Died	c. 1095 (aged about 19)
Spouse	Anna Komnene
Dynasty	Doukas
Father	Michael VII Doukas
Mother	Maria of Alania

Constantine Doukas or Ducas (Greek: Κωνσταντίνος Δούκας, Kōnstantinos Doukas, c. 1074 – c. 1095) was Byzantine junior emperor from 1074 to 1078, and again from 1081 to 1087. He was born to Emperor Michael VII Doukas and Empress Maria of Alania in about 1074, and elevated to junior emperor probably in the same year. He was junior emperor until 1078, when Michael VII was replaced by Nikephoros III Botaneiates. Because Constantine was not made junior emperor under Nikephoros III, his betrothal to Olympias, the daughter of Robert Guiscard, was broken, which Robert Guiscard used as a pretext to invade the Byzantine Empire. John Doukas forced Nikephoros to abdicate in favor of Alexios I Komnenos in 1081, and shortly afterwards Alexios elevated Constantine to junior emperor. Constantine married Alexios's daughter Anna Komnene, and remained junior emperor until 1087, when Alexios had a son, John II Komnenos. Constantine died in c. 1095.