Detecting triple systems with gravitational wave observations
  The Laser Interferometer Gravitational Wave Observatory (LIGO) has recently
discovered gravitational waves (GWs) emitted by merging black hole binaries. We
examine whether future GW detections may identify triple companions of merging
binaries. Such a triple companion causes variations in the GW signal due to (1)
the varying path length along the line of sight during the orbit around the
center of mass, (2) relativistic beaming, Doppler, and gravitational redshift,
(3) the variation of the "light"-travel time in the gravitational field of the
triple companion, and (4) secular variations of the orbital elements. We find
that the prospects for detecting the triple companion are the highest for
low-mass compact object binaries which spend the longest time in the LIGO
frequency band. In particular, for merging neutron star binaries, LIGO may
detect a white dwarf or M-dwarf perturber at signal to noise ratio of 8, if it
is within 0.4 solar radius distance from the binary and the system is within a
distance of 100 Mpc. Stellar mass (supermassive) black hole perturbers may be
detected at a factor 5x (1000x) larger separations. Such pertubers in orbit
around the merging binary emit GWs at frequencies above 1 mHz detectable by the
Laser Interferometer Space Antenna (LISA) in coincidence.
