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The actress, who dated Ledger from 2002 to 2004, took to her Instagram on April 4 to share a photo of the herself and the late actor at the Screen Actors Guild Awards in 2004.
"Happy birthday to this darling heart. We will never forget you... #heathledger #rip," Watts captioned the red carpet picture.
She also shared another photo of the duo on Instagram Story with the caption, "Beautiful Heath."
Ledger tragically passed away on January 22, 2008 at the age of 28. Just months ago, Watts also took to social media to remember Ledger on the 10th anniversary of his death.