Production sources tell us, Sarah Palin's daughter has signed on as a regular on the show. It appears she's helping to fill the void left by another famous cast member -- Farrah Abraham -- who was fired for being way too difficult. She disputes that, and says she got the axe because she was a porn star.

As you know, Bristol -- now 27 -- has 3 kids and was famously engaged to Levi Johnston. She eventually married Dakota Meyer, whom she's in the process of divorcing. She was 17 when her first kid, Tripp, was born.

Bristol will join cast members Amber Portwood, McKenzie McKee, Maci Bookout and Catelynn Lowell.

The season fires up on MTV this Fall.