Mila Kunis kissed Ashton Kutcher while vacationing with Princess Beatrice and Dave Clark in St. Tropez on June 9, 2013. Credit:

Ashton Kutcher and Mila Kunis don't typically engage in PDA, but the former That '70s Show costars threw caution to the wind when they shared a tender kiss during a couples' vacation in Saint Tropez, France, on Saturday, June 8. The loves of more than a year were joined by new pal Princess Beatrice and her boyfriend Dave Clark.

The following day, a shirtless Kutcher sunned himself while aboard a yacht with pals. The Two and a Half Men star also went scuba diving.

Kutcher, 35, and Kunis, 29, were first spotted on a double date with Beatrice, 24, and Clark in London in early May. "Princess Beatrice is obviously enamored by Hollywood and had a great time hanging out with them," a source told Us Weekly after their May 11 bar crawl.

According to another insider, it's Clark who introduced the American A-listers to his girlfriend of six years. "Bea's boyfriend, Dave, is the link," the insider explains. "He is head of relations at Virgin Galactic and that's how he met Ashton, who is going to be one of the early space travelers."

The couples bonded while Kunis was in London filming Jupiter Ascending with costar Channing Tatum. "They all get along really well," the insider tells Us. "Bea thinks Mila is awesome."