Covariant multi-galileons and their generalisation
  We find a covariant completion of the flat-space multi-galileon theory, preserving second-order field equations. We then generalise this to arrive at an enlarged class of second order theories describing multiple scalars and a single tensor, and conjecture that these are a multi-scalar version of Horndeski's most general scalar-tensor theory. 
Antonio Padilla and Vishagan Sivanesan
gr-qc astro-ph.CO hep-th