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Cameroon midfielder Song spent the last two seasons on loan at West Ham, but made just 15 appearances last season after being hampered by an ankle injury.
Song signed a five-year deal when joining Barca from Arsenal in 2012, but he struggled to make an impact at Camp Nou, starting only 29 LaLiga games for the club.
Rubin confirmed the arrival of Song on Monday, as well as the capture of Brazilian striker Jonathas from Real Sociedad.