Difference Frequency of Kilohertz QPOs Not Equal to Half the Burst   Oscillation Frequency in 4U 1636-53
  We have analyzed data obtained during two observations with the Rossi X-ray Timing 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 two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source to be 253.7 +/- 4.7 and 246.4 +/- 5.4 Hz, respectively. These values are inconsistent with being equal to 0.5 times the frequency of the 581-Hz oscillations that have been detected previously in 4U 1636-53 during type I bursts. The weighted average discrepancy is 39.5 +/- 3.5 Hz. This result shows that a simple beat-frequency interpretation of the kHz QPOs, in which the frequency of the oscillations detected during type I bursts equals the separation between the two kHz QPOs (or twice that value), is incorrect. 
Mariano Mendez, Michiel van der Klis, Jan van Paradijs (University of   Amsterdam)