Can N-body systems generate periodic gravitational waves?
  None of N-body gravitating systems have been considered to emit periodic gravitational waves because of their chaotic orbits when N=3 (or more). We employ a figure-eight orbit as a specific model for a 3-body system in order to illustrate that some of triple stars are capable of generating periodic waves. This illustration would imply that a certain class of N-body gravitating systems may be relevant to the gravitational waves generation. We show also that the total angular momentum of this 3-body system is not carried away by gravitational waves. A waveform generated by this system is volcano-shaped and thus different from that of a binary system. Finally, by evaluating the radiation reaction time scale, we give an order-of-magnitude estimate of merging event rates. The estimate suggests that figure-eight sources, which require carefully prepared initial states, may be too rare to detect. 
Takamasa Chiba, Tatsunori Imai, Hideki Asada