Leighton was previously in long-term relationships with her "Gossip Girl" co-star Sebastian Stan and artist Aaron Himelstein. Meanwhile, Adam also dated a former co-star -- Rachel Bilson -- for two years.
RELATED: Hollywood's most private couples
Aside from wedding plans, they have another aptly titled collaboration in their future: They both star in the indie feature "Life Partners." How's that for art imitating life?
Keep clicking for more photos of the pair ...
© 2015 Whalerock Industries