Page "Constantine VII" Paragraph 8

Zoe was no more successful with the Bulgarians, by whom her main supporter, the general Leo Phokas, was defeated in 917, and in 919 she was replaced as regent by the admiral Romanos Lekapenos, who married his daughter Helena Lekapene to Constantine.

Romanos used his position to advance to the ranks of basileopatōr in May 919, kaisar ( Caesar ) in September 920, and finally co-emperor in December of the same year.

Thus, just short of reaching nominal majority, Constantine was eclipsed by a senior emperor.