Y Combinator founder Paul Graham took to his blog yesterday to announce the hire of four new part-time partners--including Groupon founder and ex-CEO Andrew Mason. 

The other part-time partners, who will advise the next class of YC start-ups, include Michael Seibel, the founder of Socialcam, Steve Huffman, the co-founder of Hipmunk and Reddit, and Dalton Caldwell, the co-founder of Imeem and App.net. 

Kevin Hale, the co-founder of Wufoo, will join YC as a full-time partner. 

"We've known all these guys for years and we can already tell it will be great to work with them," Graham wrote.  Harj Taggar, the first-ever YC partner will be leaving the firm. 

But the big news here is the surprise return of Andrew Mason, the Groupon founder who was, for all intents and purposes, canned from his last gig as Groupon's CEO. He's not just becoming a part-time partner at YC, either. In a blog post on his own site--timed with Paul Graham's post--Mason had a few announcements of his own. 

"If there's a silver lining to leaving Groupon, it's the opportunity to start something new," Mason wrote. "I've accumulated a backlog of ideas over the last several years, my favorite of which I'll be turning into a new company this fall."

He doesn't offer many details about the new company, but Mason also used the post to announce a third new project that's currently in the works--a music album targeted at young people entering the workforce.

Yes. For real. (Well, so he says. But honestly, we're talking about Andrew Mason, after all.)

In his words:

I came to realize that there was a real need to present business wisdom in a format that is more accessible to the younger generation. It was with this in mind that I spent a week in LA earlier this month recording Hardly Workin', a seven song album of motivational business music targeted at people newly entering the workforce. These songs will help young people understand some of the ideas that I've found to be a key part of becoming a productive and effective employee.

We'll be sure to stay tuned.