Robert Pattinson and Kristen Stewart look glum as they relinquish a supper date in Hollywood: They're both successful movie stars, are idols for millions of teenagers and are one of the hottest couples in Hollywood. So why do Robert Pattinson and Kristen Stewart always look so miserable? The couple were snapped leaving a restaurant in Los Angeles last night with very glum expressions on their faces.

It's possible their dinner wasn't up to scratch at the Italian restaurant Ago, which is part owned by actor Robert De Niro. Or they might have had a falling out over their vino and candles at the West Hollywood trattoria. But it's more than likely that the presence of the cameras outside was enough to put them both in a mood. Kristen, 20, and Robert, 24, didn't look very impressed that their picture was being taken and refused to flash a smile in the camera's direction.

They then both hurried into a cab and sped off. The couple have tried to keep their relationship under wraps ever since they met and got together on the set of Twilight in 2008, the first film in the series based on the best-selling Stephanie Meyers books. Their relationship is an open secret but neither will talk about it and they are fiercely protective of their privacy. And while Robert looked just as moody, he does at least manage to raise a smile when caught by the cameras.

Kristen, on the other hand, is notorious for not smiling and often looks miserable whether caught candidly by photographers or posing on the red carpet. Last night though she may still have been reeling from rumours suggesting that Robert and his former Harry Potter co-star Emma Watson have become an item.

Watson and Pattinson are to appear side by side in the forthcoming remake of the 2004 film Dark Arc, which sparked reports that Kristen was jealous and was said to consider Watson ‘a threat’. But a spokeswoman for Emma said the idea of a love triangle between all three is ‘absurd and totally fabricated’.