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A long discussion has been devoted to the issue of clustering of the kHz QPOfrequency ratios in the neutron star sources. While the distribution of ratiosinferred from an occurrence of a single QPO seems to be consistent with arandom walk, the distribution based on simultaneous detections of both peaksindicates a preference of ratios of small integers. Based on the public RXTEdata we further investigate this issue for the source 4U 1636-53. Qualityfactors and rms amplitudes of both the QPOs nearly equal to the points wherethe frequencies are commensurable, and where the twin QPO detections cluster.We discuss a connection of the clustering with the varying properties of thetwo QPO modes. Assuming approximative relations for the observed correlationsof the QPO properties, we attempt to reproduce the frequency and ratiodistributions using a simple model of a random-walk evolution along theobserved frequency-frequency correlation. We obtain results which are inqualitative agreement with the observed distributions.

On the origin of clustering of frequency ratios in the atoll source 4U 1636-53