Bradley Cooper didn't wear underwear when he met US President Barack Obama.

The 'Silver Linings Playbook' actor and his girlfriend, Suki Waterhouse, met the US premiere at a state dinner last week, but he says gaining weight for a new role left him with no room for boxer shorts.

Speaking to talk show host Ellen Degeneres, he said: ''I'm prepping for new movie and I'm trying to gain weight fast, I've gained about 15 pounds in four weeks and nothing fits me anymore. I wore the tuxedo I wore to the Golden Globes and I just sort of threw it in the bag and went to Washington D.C.

''I'm getting dressed, like, 10 minutes before I was supposed to go and literally the pants go up to like here [halfway up his thigh].

''So I had to go commando because there was no room at all. I did. I had to... So it was really great. And you have to walk up the stairs and it was a nightmare. But I made it through.''

Bradley, 39, is nominated for Best Supporting Actor for his role in 'American Hustle' at the Oscars on March 2 - his second year nominated for the award in a row - and is having trouble deciding between taking his sister and his mother with him, so appealed to Ellen, the awards' host, for help.

He explained: ''I'm trying to take more than one person. My sister's coming from North Carolina ... How can I [choose?]. Maybe they'll sit them at the top, maybe there's some seats at the top?''

Ellen initially sounded doubtful, but later added: ''I'll see what I can do for you.''