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The vacancy next to Di Resta is one of only three across the grid for the 2013 season, which begins with the Australian Grand Prix in Melbourne on March 17.
French driver Jules Bianchi, the team's third driver in 2012, was expected to be announced as Di Resta's teammate.
However, he was present at the launch of Ferrari's new F1 car on Friday as he is a member of the Italian team's academy.
Former driver Adrian Sutil and ex-Sauber man Kamui Kobayashi have also been linked with the role.
Force India team principal Robert Fearnley refused to be drawn on the identity of the new driver at the launch, telling reporters: "We do not want to confuse the two stories."
The VJM06 is the third of four cars unveiled so far for the 2013 season that have jettisoned the stepped nose design that featured widely in last year's world championship.
The Indian team finished seventh in last season's constructor championship, but saw German driver Nico Hulkenburg leave to join rivals Sauber at the end of the year.