Estudiantes defender Santiago Vergini has re-joined Sunderland on loan.
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The 26-year old made 14 appearances for the Black Cats following a loan move in January.
After an unconvincing start, the Argentine was a part of the team which finished the season strongly to preserve Sunderland's place in the Premier League.
Vergini, a central defender who also played at right-back for the Wearsiders, told the club's official website: "I'm very thankful to the club for giving me this opportunity. I am very happy to be here once again. I'm also very happy to see so many recognisable faces.
"The challenge for me is to become part of this team and offer all I can so we have a good season. The feeling that I've had here at this club has been very good. It's a big club."
Vergini becomes Sunderland's sixth signing of the summer, following the arrivals of goalkeeper Costel Pantilimon, defenders Billy Jones and Patrick van Aanholt and midfielders Jordi Gomez and Jack Rodwell.
Meanwhile, Sunderland midfielder El-Hadji Ba has joined Bastia on a season-long loan.
The 21-year old Frenchman, who joined last summer from Le Havre, returns to Ligue 1 and a side managed by former Chelsea midfielder Claude Makelele after figuring in just three first-team matches for the Black Cats.
Ba tweeted: "I want just say thanks to (Paolo) Di Canio, Gus Poyet, the staff and players for all they have taught me. Fans were very nice I wish you the best."