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The 559 Hz black-widow pulsar PSR J1555-2908, originally discovered in radio,is also a bright gamma-ray pulsar. Timing its pulsations using 12 yr ofFermi-LAT gamma-ray data reveals long-term variations in its spin frequencythat are much larger than is observed from other millisecond pulsars. Whilethis variability in the pulsar rotation rate could be intrinsic "timing noise",here we consider an alternative explanation: the variations arise from thepresence of a very-low-mass third object in a wide multi-year orbit around theneutron star and its low-mass companion. With current data, thishierarchical-triple-system model describes the pulsar's rotation slightly moreaccurately than the best-fitting timing-noise model. Future observations willshow if this alternative explanation is correct.

Is the black-widow pulsar PSR J1555-2908 in a hierarchical triple system?