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Freddie Ljungberg passed a medical examination and agreed to represent the Glasgow club until the end of the season.
The 33-year-old Swede's debut could take place already this Sunday in the crucial derby against Glasgow Rangers.
"During the recent training sessions, Freddie looked great. I got a great impression," said Celtic coach Neil Lennon. "He will be a great asset to the team."
For the past year and a half, Ljungberg has been in the United States, but he is most famous for representing the London Arsenal team.
