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We present results of an analysis of all 480 ks of Rossi X-ray TimingExplorer Proportional Counter Array data obtained from 17 May 1998 to 11October 1998 on the luminous low mass X-ray binary GX 13+1. We analysed thespectral properties in colour-colour diagrams (CDs) and hardness-intensitydiagrams (HIDs) and fitted the power spectra with a multi-Lorentzian model. GX13+1 traces out a curved track in the CDs on a time scale of hours, which isvery reminiscent of a standard atoll track containing an island, and lower andupper banana branch. However, both count rate and power spectral propertiesvary along this track in a very unusual way, not seen in any other atoll or Zsource. The count rate, which varied by a factor of ~1.6, along a given trackfirst decreases and then increases, causing the motion through the HIDs to bein the opposite sense to that in the CD, contrary to all other Z and atollsources. Along a CD track, the very low frequency noise uniquely decreases inamplitude from ~5 to ~2% (rms). The high frequency noise amplitude decreasesfrom ~4% to less than 1% and its characteristic frequency decreases from ~10 to\~5 Hz. The 57-69 Hz quasi-periodic oscillation (QPO) found earlier is alsodetected, and no kHz QPOs are found. In addition the entire track shows secularmotion on a time scale of about a week. The average count rate as well as theamplitude of the very low frequency noise correlate with this secular motion.We discuss a possible explanation for the peculiar properties of GX 13+1 interms of an unusual orientation or strength of a relativistic jet.Comment: 15 pages, 13 figures. Accepted for publication in A&

Peculiar spectral and power spectral behaviour of the LMXB GX 13+1