Degenerate higher order scalar-tensor theories beyond Horndeski up to   cubic order
  We present all scalar-tensor Lagrangians that are cubic in second derivatives of a scalar field, and that are degenerate, hence avoiding Ostrogradsky instabilities. Thanks to the existence of constraints, they propagate no more than three degrees of freedom, despite having higher order equations of motion. We also determine the viable combinations of previously identified quadratic degenerate Lagrangians and the newly established cubic ones. Finally, we study whether the new theories are connected to known scalar-tensor theories such as Horndeski and beyond Horndeski, through conformal or disformal transformations. 
Jibril Ben Achour, Marco Crisostomi, Kazuya Koyama, David Langlois,   Karim Noui, Gianmassimo Tasinato
hep-th astro-ph.CO gr-qc