Can't believe I did not answer that question, but jasonpatterson is correct.
In the early stage of the mission, i.e. after arrival at 6 August (yeah! only 3 more weeks!), there are bound orbits, which last about a week for one revolution. The latest estimate of the mass that I could find was MCG ≈ 3.14±0.21 ⨯ 1012 kg.
However, as soon as the comet starts to become more active, the solar panels of the satellite are going to act as sails, and thus there can no longer be bound orbits, as the spacecraft will be pushed out by the outgassing. Then the "orbits" changes to "flybys" where Rosetta will be flying on hyperbolic "orbits" with a burn at the end of each, to send it back to the comet.
Somehow I cannot find the pictures of the orbit but you can google that.