Primordial black holes (PBHs), if trapped in neutron stars, emit a characteristic continuous, quasiperiodic gravitational wave (GW) signal as they orbit inside the host star. We identify a specific and qualitatively new feature of these signals, namely quasiperiodic beats caused by the precession of noncircular PBH orbits. We demonstrate numerically and analytically that the beat frequency depends rather sensitively on the neutron star structure, so that hypothetical future observations with next-generation GW detectors could provide valuable constraints on the nuclear equation of state.

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• General relativity
• Gravitational waves
• Classical black holes
• Neutron stars & pulsars