Saturday's event showed off a new red and white livery, with Alfa Romeo branding prominent on the engine cover, and a redesigned logo.
Giovinazzi is still part of the Sauber line-up, with the Italian taking on the role of third driver and set to participate in a number of Friday practice sessions during the 2018 season having raced twice at the start of this year in place of the injured Wehrlein.
Sauber, meanwhile, had Marcus Ericsson angling for a seat despite not scoring any points in either of the last two seasons, relying heavily on his links to the team's owners. The title sponsorship deal means the team will be called Alfa Romeo Sauber F1 and the multi-year partnership comes with technical and commercial benefits.More news: United States tourists held for baring buttocks at Thai temple
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Fiat Chrysler (FCA) have invested billions of euros since 2014 to develop new Alfa Romeo models that would stand up against German premium rivals and shed a reputation for poor quality and service that has hit sales in the past decade.
Formula One chairman Chase Carey and former Ferrari boss Jean Todt, president of the governing FIA, both attended the presentation.
Leclerc had always been tipped for one of the Sauber seats after storming to the Formula 2 title, finishing 72 points ahead of Artem Markelov.
"For now I'm a Ferrari fan, but if Alfa Romeo Sauber do well that would of course be very welcome", Marchionne said.