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We investigate the long-term variability of the K$\alpha$ line of iron in thespectra of two Ultra Compact X-ray Sources (UCXBs) with C/O-rich donors. Werevisit archival observations by five different X-ray telescopes, over a~twenty year period. Adopting physically motivated models for the spectralcontinuum, we probe the long-term evolution of the source emission in aself-consistent manner enabling physical interpretation of potentialvariability in the primary X-ray emission continuum and/or any emission linesfrom reflection off the accretion disk. We find that the spectral shape andflux of the source emission (for both objects) has remained almost constantthroughout all the observations, displaying only minor variability in somespectral parameters and the source flux (largest variation is a ~25% drop inthe flux of Swift J1756.9-2508). We note a striking variability of the FeK$\alpha$ line which fluctuates from a notable equivalent width of ~66-100 eVin 4U 1543-624 and ~170 eV in Swift J1756.9-2508, to non-detections with upperlimits of 2-8 eV. We argue that the disappearance of the iron line is due tothe screening of the Fe K$\alpha$ line by the overabundant oxygen in theC/O-rich UCXBs. This effect is cancelled when oxygen becomes fully ionized inthe inner disk region, resulting in the variability of the Fe K$\alpha$ line inan otherwise unaltered spectral shape. This finding supports earlierpredictions on the consequences of H-poor, C/O-rich accretion disk onreflection induced fluorescent lines in the spectra of UCXBs.

Disappearance of the Fe K$\alpha$ emission line in Ultra Compact X-ray Binaries 4U 1543-624 and Swift J1756.9-2508