Mick Jagger in studio with Ringo Starr.

Rolling Stones lead singer Mick Jagger has been working in a recording studio on a project with Ringo Starr and Joe Walsh, while Rolling Stones lead guitarist Ronnie Wood joined Paul McCartney on stage during a concert for Paul McCartney's son James.
On March 8, 2013, Walsh tweeted a group photo of him in a recording studio with several people, including Jagger, Starr, longtime Rolling Stones producer Don Was, blues musician Keb' Mo' and bass player Tal Wilkenfeld. Walsh said in the accompanying message: "Cooking up something here at Capitol Records! I think you'll like it."
It has not yet been announced what this project is or when it will be released.
Throughout March 2013, Wilkenfeld posted a series of vague tweets about the recording sessions:

“Thursdays in the office are usually pretty mellow. Like today… when I wrote a song with Bill Withers, Mick Jagger, Keb Mo & Joe Walsh. LOL."
“If I start thinkin about what's goin down in the studio tomorrow, I'll never sleep.”
“Usual day at the office... ya know, recording with Mick Jagger, Ringo Starr & Joe Walsh.”
“Gr8 fun recordin today with @JoeWalsh @kebmomusic @rrtfb @akadrjohn ,Jim Keltner & Mike Finnigan! Don Was producing. Dunno who is now! Lol.”