text: Bottas, on whom Williams have a contractual hold, is Ferrari's number one option should they decide not to retain Kimi Raikkonen alongside Sebastian Vettel.
  Sources close to the deal say Ferrari and Williams have discussed Bottas's transfer but have no agreement yet.
  Williams view Button as a leading candidate should Bottas leave.
  Should the 2009 world champion join Williams in 2016, it would bring his career full circle.
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  He made his F1 debut for Williams in 2000 before moving on to Benetton/Renault, BAR - which became Honda and then Brawn - and finally McLaren in a career in which he has won 15 grands prix.
  McLaren have not yet decided who they will pick to drive alongside Fernando Alonso in 2016.
  The team have a contractual option to retain Button, which means that the 2009 world champion cannot join another team unless McLaren decide not to keep him - or fail to decide until after whatever date is set in the contract as the team's deadline to do so.
  Their other options are their Danish reserve driver Kevin Magnussen, who raced for McLaren alongside Button in 2014, and the Belgian rising star Stoffel Vandoorne, who is leading the GP2 championship. McLaren have contracts with both men.
  Williams said they could not comment on driver contracts and Button said he had not yet considered his future.
  "I've had a lot of questions thrown at me about what I am doing next year but I have not thought about anything, seriously," the 35-year-old said.
  "I just have to get my head down and concentrate on improving the car and then in a couple of months we will sit down and talk about next year."
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  Bottas, on whom Williams have an option for the next two seasons, said he "did not know" whether he would be at Williams next season.
  "Nothing is confirmed," the Finn said. "We have to wait and see. As a driver, you want to know as soon as possible but sometimes you have to wait."
  Although Ferrari have pursued various options, sources close to the team say they have decided to put off a decision on Raikkonen's future until later in the summer.
  Ferrari president Sergio Marchionne said last month that Raikkonen's "future is in his own hands" amid concerns at the team that he is not performing well enough compared to team-mate Sebastian Vettel.
  These come on the back of his poor season in 2014 during which he was comprehensively out-performed by former team-mate Alonso.
  Ferrari technical director James Allison said this week in an interview with an Italian magazine that Raikkonen was as fast as Vettel but that his results were being affected by the fact he was making more mistakes.
  Raikkonen said: "I don't know any more than I knew at the start of the year. For me, the whole thing hasn't changed for months. If I knew I would easily say. Unfortunately I don't know.
  "Somehow I end up every year in the same position, and it makes no difference if I have a contract or not. There is no contract, but there is an option now.
  "I have made it very clear to the team what I would like to happen, and it is to be here next year."
  If Bottas leaves and Button stays at McLaren, Williams may still look to McLaren's driver pool for a partner for Felipe Massa, on whom they have an option and is expected to stay.
  Magnussen and Vandoorne could both be of interest to Williams.
target: Jenson Button has emerged as a potential target for Williams next season if they lose Valtteri Bottas.