Cardiff midfielder Aron Gunnarsson has signed a new two-year contract extension with the club.
Celtic have signed defender Dedryck Boyata from Manchester City on a four-year contract.
St Mirren defender David van Zanten has signed a one-year extension to his contract which will keep him at the club until the summer of 2014.
Van Zanten made his 20th SPL start of the season in the 2-1 home win over Inverness last week and that triggered a clause in his current deal.
The Irishman, who returned for a second spell with the Buddies on a free transfer from Hamilton in August 2010, was a happy man after putting pen to paper.
He told the club's official website: "To be honest I'm absolutely delighted. When I signed my last contract I had the clause that if I started 20 league games it would trigger another year for me.
"That has been in my thoughts all season, trying to get to these 20 games as soon as possible, so I'm really pleased to know that I'll be spending my ninth season with the club now.
"I would love to continue playing with St Mirren for as long as I can but that basically all depends on my performances.
"So I hope to keep the standard of my performances up to a good level for as long as I can and if I do I hope to be with the club for many more years to come."