According to Autocar, rumors of Mercedes-Benz purchasing a major stake in Aston Martin don't hold water. Aston's current owners are reportedly not interested in selling all or part of their British sportscar marque to anybody at this time. The possibility still exists that Aston will go public within the next five-or-so years, meaning that just about anyone could own a piece of the tasty Aston Martin pie in the future.
Dreams of a beautifully styled Aston Martin powered by a glorious AMG engine are still possible, though, as Aston's current owners have been in talks with multiple other automakers over parts-sharing opportunities. Major technical components could be bought or developed in collaboration with a company that's got plenty of R&D acumen, and MB/AMG certainly qualifies as such. Although a possible synergy does exist between the two brands, nothing beyond collaboration and the sharing of technical know-how seems likely.