Rosberg and Nakajima both followed their fathers' footsteps into F1

The Williams-Toyota team will retain Nico Rosberg and Kazuki Nakajima as the team's drivers for the 2009 season.

Nico Hulkenberg, currently leading the Formula 3 Euroseries, will continue as the official test driver.

Germany's Rosberg, 23, moved to 12th in the world championship standings after his second-place finish at the Singapore Grand Prix on Sunday.

Team principal Frank Williams said: "I am delighted to be able to maintain continuity with our drivers."

Rosberg, the son of former F1 world champion Keke Rosberg, has made steady progress since his F1 debut at the 2006 Bahrain Grand Prix.

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Frank Williams described the German as "one of the most capable drivers in the sport", adding that Japan's Nakajima was "getting stronger all the time".

Nakajima, 23, made his debut when he replaced the retired Alexander Wurz in last season's final race, and is also following in the footsteps of his father Satoru Nakajima by lining up on the grid.