Williams have signed ex-McLaren tester Alexander Wurz as their third driver for the 2006 Formula One season.
Wurz will understudy Webber and Rosberg
The Austrian was left without a role at McLaren when they signed Gary Paffett. Team boss Sir Frank Williams said: "I am delighted with this agreement."
Williams' disappointing form in 2005 means they can use a third driver on the Friday of race weekends this year.
Sir Frank added: "We all believe he will bring significant improvements to the team's performance."
Willams came close to offering India's Narain Karthikeyan the role but opted for experience rather than funding.
Signing Wurz gives them the chance to benefit from his extensive knowledge of McLaren's 2005 race-winning car.
Wurz, also understudy to racers Mark Webber and Nico Rosberg, said: "Ever since I met Frank Williams in 1996, I have wanted to drive for his team - and I am delighted that the opportunity has now arisen.
"I feel full of energy to be part of Williams' 2006 season attack and look forward to working closely with Mark and Nico.
"They are quick drivers, and I believe that together we will achieve great results.
"The testing and Friday driver role is a responsibility I am familiar with, and I will throw all my experience into it. I cannot wait to start."