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Pairs of quasi-periodic oscillations (QPOs) at kilohertz frequencies are acommon phenomenon in several neutron-star low-mass X-ray binaries. Thefrequency separation of the QPO peaks in the pair appears to be constant inmany sources and directly related to the neutron star spin frequency. However,in Sco X-1 and possibly in 4U 1608-52, the frequency separation of the QPOsdecreases with increasing inferred mass accretion rate. We show that thecurrently available Rossi X-ray Timing Explorer data are consistent with thehypothesis that the frequency separations in all sources vary by amountssimilar to the variation in Sco X-1. We discuss the implications for models ofthe kilohertz QPOs.Comment: 8 pages, 3 b&w figures and 1 color figure; to appear in the  Astrophysical Journal Letter

The Beat-Frequency Interpretation of Kilohertz Quasi-periodic Oscillations in Neutron Star Low-Mass X-Ray Binaries