Robert Pattinson's biggest fear has been realized.

On July 17, the 26-year-old actor's girlfriend and Twilight costar, Kristen Stewart, 22, was caught in a series of steamy rendezvous with her Snow White and the Huntsman director, Rupert Sanders, 41 -- and only Us Weekly has the photographic evidence.

PHOTOS: Kristen Stewart and Robert Pattinson's undercover PDA

Sanders -- married to British Vogue model Liberty Ross, 33 -- has two children, ages 7 and 5. During his marathon makeout session with Stewart, "it seemed like they couldn't get enough," according to one photographer.

PHOTOS: Kristen Stewart's grumpiest expressions

Reps for both Stewart and Sanders had no comment.

For more details on Stewart's affair -- from her initial interactions with Sanders to how her infidelity will affect her Twilight Saga: Breaking Dawn -- Part 2 promotional duties with Pattinson -- pick up the new issue of Us Weekly, out Friday!

This article originally appeared on Kristen Stewart Cheats on Robert Pattinson With Director Rupert Sanders