A modified "Type I migration" model for propeller moons in Saturn's   rings
  We propose a mechanism for the observed non-keplerian motion (Tiscareno et al. 2010, ApJL) of "propeller" moons embedded in Saturn's rings. Our mechanism, in which radial variations in surface density -- external to, and unaffected by, the embedded moon -- result in an equilibrium semimajor axis for the moon due to "Type I" angular momentum exchange (Crida et al. 2010, AJ), provides a good fit to the observations. Future observations should distinguish between our model and others recently proposed. 
Matthew S. Tiscareno
astro-ph.EP