Cairo, who was signed by Iain Dowie but featured way more for City Reserves than he did the first team in the 2007/2008 season, has impressed after a trial.
Cairo was a free agent after his contract with NAC Breda ended at the end of last season.
Speaking about Cairo to the Heracles club website, Director Nice-Jan Hoogma said: "He has in recent weeks had a fair chance to prove himself here and show that he will improve our squad. "He grabbed that chance and so Ellery has signed a contract for the remainder of the season."