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Zac Efron and Nina Dobrev were spotted looking pretty chummy with each other this week ... getting close as they chatted on a yacht during their sun-soaked getaway with friends in Sardinia.

Check out the pics ... the actors appeared comfortable with each other -- standing quite close -- as they let out a laugh while their vessel sailed into Porto Cervo at sunset. Backgrid

It wasn't all fun and games ... earlier in the day, Nina looked somber as she was comforted by her friend Keleigh Sperry on the upper deck, Zac nowhere in sight.

It's not clear what may have upset her, but as you know, she and her longtime partner Shaun White just called off their engagement to the astonishment of fans. Gallery Launch Gallery

As we

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