Which Byzantine empress ruled when Charlemagne was crowned emperor?

Irene of Athens ruled the Byzantine Empire when Charlemagne was crowned emperor in Rome on Christmas Day in 800. At that time, Irene was empress regnant and sole ruler of the Eastern Roman Empire, a position she held from 797 until 802.

Irene had earlier served as empress consort, regent, and co-ruler. After the death of her husband, Leo IV, she governed on behalf of their son, Constantine VI. In 797, she arranged Constantine’s deposition and blinding, then took the throne in her own right. Her reign was unusual because she was a woman ruling alone in a political tradition that generally expected an emperor to be male.

Her relationship to Charlemagne became politically important. Some Western sources portray Charlemagne’s coronation as filling a vacant imperial office because Irene was a woman, while Byzantine authorities did not accept the Western claim as legitimate. Irene is also remembered for supporting the restoration of the veneration of religious images at the Second Council of Nicaea in 787. She was succeeded by Nikephoros I in 802.