Precession can modulate pulsar frequencies in much the same way as the Doppler shift caused by orbital motion with a planetary companion. We derive a relation among observables for situations where a companion is suspected, which if satisfied, would make it difficult to distinguish planetary modulation of pulsar frequency from that caused by free precession of an isolated pulsar. Conversely, if the relation cannot be satisfied, precession is ruled out, and a planetary hypothesis thereby strengthened.
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