Williams have confirmed Narain Karthikeyan as their fourth driver this season.
The Indian driver, who made his Formula One debut last season with the Jordan team, will test for Williams but he will not drive in Friday practice of Grand Prix weekends - a role reserved for the experienced Alex Wurz.
"Narain's brief period in the car in December, during which he provided valuable feedback, made a very distinct impression on our engineers," Frank Williams commented.
"Having a year's GP racing and testing to his credit is also advantageous for us, so I am pleased we can now formally confirm his appointment to the team."
Karthikeyan himself added: "It has taken some while to resolve what I would be doing for 2006, but having tested for Williams late last year, joining the team was the best option for me and I am delighted that we can finally confirm this today.
"I am looking forward to testing the FW28 and making a contribution to Williams' competitiveness this season."