  We search for the Fe K${\alpha}$ line in spectra of Ultra Compact X-ray
Binaries (UCXBs). For this purpose we have analyzed XMM-Newton observations of
five confirmed UCXBs. We find that the object 2S 0918-549 - whose optical
spectrum bears tentative signatures of a C/O accretion disk - is devoid of any
emission features in the 6-7 keV range, with an upper limit of less than 10 eV
for the equivalent width (EW) of the iron line. 4U 1916-05 - whose optical
spectrum is consistent with reflection from a He-rich accretion disk - exhibits
a bright broad iron emission line. This behavior is in agreement with the
theoretical predictions presented in Koliopanos, Gilfanov and Bildsten (2013).
Namely, we expect strong suppression of the Fe K${\alpha}$ emission line in
spectra originating in moderately bright (LogLx less than $\approx$ 37.5) UCXBs
with C/O or O/Ne/Mg-rich donors. On the other hand the EW of the iron line in
spectra from UCXBs with He-rich donors is expected to retain its nominal value
of $\approx$ 100 eV. Our analysis also reveals a strong Fe K${\alpha}$ line in
the spectrum of 4U 0614+091. This detection points towards a He-rich donor and
seems to be at odds with the source's classification as C/O-rich. Nevertheless,
a He-rich donor would explain the bursting activity reported for this system.
Lastly, based on our theoretical predictions, we attribute the lack of a strong
iron emission line - in the two remaining UCXB sources in our sample (XTE
J1807-294, 4U 0513-40) - as an indication of a C/O or O/Ne/Mg white dwarf
donor. From the upper limits of the Fe K${\alpha}$ line EW in 4U 0513-40, 2S
0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-iron
ratio,O/Fe$\ge$ 10$\times$[O/Fe]$_{\odot}$.
