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We have analyzed the X-ray spectra of all known Ultra Compact X-ray Binaries(UCXBs), with the purpose of constraining the chemical composition of theiraccretion disk and donor star. Our investigation was focused on the presence(or absence) of the Fe K${\alpha}$ emission line, which was used as the probeof chemical composition of the disk, based on previously establishedtheoretical predictions for the reflection of X-ray radiation off the surfaceof C/O-rich or He-rich accretion disks in UCXBs. We have contrasted the resultsof our spectral analysis to the history of type I X-ray bursts from thesesystems, which can also indicate donor star composition. We found that UCXBswith prominent and persistent iron K${\alpha}$ emission also featured repeatbursting activity. On the other hand, the UCXBs for which no iron line wasdetected, appear to have few or no type I X-ray bursts detected over more thana decade of monitoring. Based on Monte Carlo simulations, demonstrating astrong correlation between the Fe K${\alpha}$ line strength and the abundanceof C and O in the accretion disk material and given the expected correlationbetween the H/He abundance and the recurrence rate of type I X-ray bursts, wepropose that there is a considerable likelihood that UCXBs with persistent ironemission have He-rich donors, while those that do not, likely have C/O orO/Ne/Mg-rich donors. Our results strongly advocate for the development of moresophisticated simulations of X-ray reflection from hydrogen-poor accretiondisks.

The chemical composition of the accretion disk and donor star in Ultra Compact X-ray Binaries: A comprehensive X-ray analysis