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.