Observational Constraints on Planet Nine: Cassini Range Observations
  We significantly constrain the sky position, distance, and mass of a possible additional, distant planet in the solar system by examining its influence on the distance between Earth and the Cassini Spacecraft. Our preferred region is approximately centered on (RA, Dec) = ($40\arcdeg$, $-15\arcdeg$), extending approximately 20 degrees in all directions. 
Matthew J. Holman and Matthew J. Payne
astro-ph.EP