Tesla rose to $398.35 during Thursday’s trading, though its shares were still down 11% year to date. SpaceX shares gained 0.6% to $140.63.

Bloomberg, citing people familiar with the matter, reported that the companies could announce a deal as soon as December. A merger would be the largest corporate combination in history, according to the report. The companies’ boards and advisers are negotiating the terms and the structure of the combined entity.

Representatives for Tesla and SpaceX did not immediately respond to Bloomberg’s requests for comment, the news outlet reported.