Reps for Leighton and Adam did not return calls seeking comment.
For a pair of pretty faces from the small screen -- Leighton played Blair Waldorf on "Gossip Girl" while Adam starred as Seth Cohen on "The O.C." -- you'd expect tongues to be wagging all over town about their courtship. However, it's been quite the opposite.
They've managed to stay out of the spotlight for most of their romance. While they have been photographed together a handful of times –- from dinner dates, movie outings and trips to New York -- the duo have never commented on their relationship.
In February, news broke that they were an item. At the time, a source told Us Weekly that the pair had been "secretly dating" after being friends for several years. They previously worked together on the 2011 film "The Oranges."
They gave a subtle confirmation of their status when they were spotted arm-in-arm while attending the June 26 premiere of Adam's film "Some Girl(s)." They've also been spotted holding hands and engaging in other mild PDA on date nights, coffee runs and dog walks over the summer and into the fall.
No word yet on whether they've set a date for the rumored nuptials.
Keep clicking for more on the couple's dating history ...