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We have analyzed data obtained during two observations with the Rossi X-rayTiming Explorer on January 5 and 8, 1997, of the low-mass X-ray binary (LMXB)and atoll source 4U 1636-53. We measure the frequency separation of the twosimultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source tobe 253.7 +/- 4.7 and 246.4 +/- 5.4 Hz, respectively. These values areinconsistent with being equal to 0.5 times the frequency of the 581-Hzoscillations that have been detected previously in 4U 1636-53 during type Ibursts. The weighted average discrepancy is 39.5 +/- 3.5 Hz. This result showsthat a simple beat-frequency interpretation of the kHz QPOs, in which thefrequency of the oscillations detected during type I bursts equals theseparation between the two kHz QPOs (or twice that value), is incorrect.Comment: Accepted for publication in The Astrophysical Journal Letters. 4  pages plus 1 postscript figur

Difference Frequency of Kilohertz QPOs Not Equal to Half the Burst Oscillation Frequency in 4U 1636−53