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Joe Jonas looked buff in Hollywood Tuesday ... he was almost busting out of a tight checkered shirt!

The singer is back in LA after hitting up Chicago for radio station B96's annual summer bash. Jonas has been unusually lady-less lately; the Jonas brother is known as the casanova of the three, having dated Taylor Swift, Ashley Greene, Demi Lovato and more.

But it's younger bro Nick Jonas, 18, who is making headlines thanks to his summer romance with Australian singer Delta Goodrem, 26!

What do you think -- does Nick look lonely?