Gravitational wave background from hybrid topological defects
  We investigate the spectrum of stochastic gravitational wave background generated by hybrid topological defects: domain walls bounded by strings and monopoles connected by strings. Such defects typically decay early in the history of the universe, and their mass scale is not subject to the constraints imposed by microwave background and millisecond pulsar observations. Nonetheless, the intensity of the gravitational wave background from hybrid defects can be quite high at frequencies above $10^{-8} Hz$, and in particular in the frequency range of LIGO, VIRGO and LISA detectors. 
Xavier Martin and Alex Vilenkin (Tufts University)