Rachel McAdams, Michael Sheen make red carpet debut at Cannes

(CBS) - Things may be getting serious between Rachel McAdams and Michael Sheen. The actors, who costar in the Woody Allen flick, "Midnight in Paris," made their red-carpet debut together at the Cannes Film Festival.

McAdams, dressed in an embroidered Marchesared silk gown, and Sheen were all smiles as they posed for photographers at the premiere for their movie on Tuesday night.

Pictures: 2011 Cannes Film Festival

"She's a genuinely lovely lady as well as being stunningly beautiful and very talented," the Welsh actor said of McAdams to "Entertainment Tonight Canada."

The pair, who have been romantically linked since last fall, were recently at the center of engagement rumors after McAdams was spotted wearing a ring on her wedding finger.  But a rep for Sheen shot down speculation, telling  People magazine that the two are not engaged.

McAdams, 34, previously had relationships with actors Ryan Gosling and John Lucas, while Sheen, 42, once dated Kate Beckinsale, with whom he has a 12-year-old daughter.