Page "Constantius II" Paragraph 23

However, Constantius realised that too many threats still faced the Empire, and he could not possibly handle all of them by himself, so on 6 November 355, he elevated his last remaining relative, Julian, to the rank of Caesar.

A few days later, Julian was married to Helena, the last surviving sister of Constantius.

Not long after Constantius sent Julian off to Gaul.