Rapid variability of the kHz-QPO frequency in 4U1608-52
  We investigate the variability of the QPO frequency in 4U1608-52 on very short time scales of a few seconds. We detect changes in frequency as fast as 0.5 Hz/s, which is more than ten times the fastest change reported for this object. Using a structure function analysis, we are able to detect variability on time scales as short as ~4 seconds. We do not detect any time asymmetry in the QPO-frequency history. These results have important consequences on the measurement of the QPO coherence. 
S. Paltani (OAMP, France), D. Barret (CESR, France), J.F. Olive (CESR,   France), G. K. Skinner (CESR, France)