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To confirm the nature of the donor star in the ultra-compact X-ray binarycandidate 47 Tuc X9, we obtained optical spectra (3,000$-$10,000 {\AA}) withthe Hubble Space Telescope / Space Telescope Imaging Spectrograph. We find nostrong emission or absorption features in the spectrum of X9. In particular, weplace $3\sigma$ upper limits on the H$\alpha$ and HeII $\lambda 4686$ emissionline equivalent widths $-$EW$_{\mathrm{H\alpha}} \lesssim 14$ {\AA} and$-$EW$_{\mathrm{HeII}} \lesssim 9$ {\AA}, respectively. This is much lower thanseen for typical X-ray binaries at a similar X-ray luminosity (which, for$L_{\mathrm{2-10 keV}} \approx 10^{33}-10^{34}$ erg s$^{-1}$ is typically$-$EW$_{\mathrm{H\alpha}} \sim 50$ {\AA}). This supports our previoussuggestion (by Bahramian et al.) of an H-poor donor in X9. We perform timinganalysis on archival far-ultraviolet, $V$ and $I$-band data to search forperiodicities. In the optical bands we recover the seven-day superorbitalperiod initially discovered in X-rays, but we do not recover the orbitalperiod. In the far-ultraviolet we find evidence for a 27.2 min period (shorterthan the 28.2 min period seen in X-rays). We find that either a neutron star orblack hole could explain the observed properties of X9. We also perform binaryevolution calculations, showing that the formation of an initial black hole /He-star binary early in the life of a globular cluster could evolve into apresent-day system such as X9 (should the compact object in this system indeedbe a black hole) via mass-transfer driven by gravitational wave radiation.

HST spectrum and timing of the ultra-compact X-ray binary candidate 47 Tuc X9