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Abridged: We present a spectral and temporal analysis of the Cathedral QPOdetected in the power density spectra of the microquasar XTE J1859+226 obtainedwith RXTE. This type of QPO manifests as two peaks with similar amplitudes andharmonically related centroid frequencies (~3 and ~6 Hz). The amplitude of the~3 Hz feature varies in anticorrelation with the count rate, by about ~50. The~6 Hz feature shows a slight increase (~7%) of its amplitude with count rate.The RMS-spectra of the two peaks are quite different. The ~3 Hz feature issofter than the other one, and shows a cut-off at an energy of ~6 keV while theRMS of the 6 Hz increases up to at least 20 keV. We also study the bicoherence,$b^2(\mu,\nu)$. The values b^2(~3,~3) and b^2(~6,~6) are rather high andsimilar to the type C QPOs of GRS 1915+105. By comparison with the lattersource the fact that the bicoherence of the ~3 Hz feature is higher than thatof the other peak, would tend to indicate that the ~3 Hz is the fundamentalQPO. The value of b^2(~3,~6)$ is, however, low and therefore indicates abehavior that is different than that seen in GRS 1915+105. We suggest that,rather than pure harmonics, we may see different modes of the same underlyingphenomenon competing to produce QPOs at different frequencies.

The puzzling harmonic behavior of the Cathedral QPO in XTE J1859+226