Context : Moderately metal-poor inner bulge globular clusters are relics of a generation of long-lived stars that formed in the early Galaxy .
Terzan 9 , projected at 4 \aas@@fstack { \circ } 12 from the Galactic center , is among the most central globular clusters in the Milky Way , showing an orbit which remains confined to the inner 1 kpc .

Aims : Our aim is the derivation of the cluster ’ s metallicity , together with an accurate measurement of the mean radial velocity .
In the literature , metallicities in the range between - 2.0 < [ Fe/H ] < - 1.0 have been estimated for Terzan 9 based on color-magnitude diagrams and CaII triplet ( CaT ) lines .

Methods : Given its compactness , Terzan 9 was observed using the Multi Unit Spectroscopic Explorer ( MUSE ) at the Very Large Telescope .
The extraction of spectra from several hundreds of individual stars allowed us to derive their radial velocities , metallicities , and [ Mg/Fe ] .
The spectra obtained with MUSE were analysed through full spectrum fitting using the ETOILE code .

Results : We obtained a mean metallicity of [ Fe/H ] \approx -1.10 \pm 0.15 , a heliocentric radial velocity of v ^ { h } _ { r } = 58.1 \pm 1.1 km s ^ { -1 } , and a magnesium-to-iron [ Mg/Fe ] = 0.27 \pm 0.03 .
The metallicity-derived character of Terzan 9 sets it among the family of the moderately metal-poor Blue Horizontal Branch clusters HP 1 , NGC 6558 , and NGC 6522 .

Conclusions :