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That's going to be one funny baby!

It's Always Sunny in Philadelphia stars Charlie Day and Mary Elizabeth Ellis welcomed a son on Thursday in L.A., her rep confirms to Us Weekly.

This is the first child for Day, 35, and Ellis, 32. The Horrible Bosses star and Ellis wed in March 2006.  Russell Wallace Day weighed in at 7ib. 13 oz. and was 21.5 inches long.

The duo's It's Always Sunny costar Glenn Howerton also became a father for the first time in September. Howerton admitted that their son Miles' moniker was a bit of a last minute decision.

"My wife and I thought it would be a good idea to wait to see the baby first before we actually decided what his name would be. That wasn't a good idea. We thought it would be a good idea to get a sense of the guy," he told Conan in September.'

"Then we saw him and he looked like a tiny little baby alien grandpa creature and I was like, 'I'm not getting any information from this -- at all,'" the actor laughed.