Ancient, eclipsing, tidally-locked: A blue lurker progenitor in the population of extreme-velocity star candidates

Many extreme velocity candidate stars have been found based on $\mathit{\text{Gaia}}$ astrometry, but need spectroscopic confirmation. We select late-type hypervelocity star (HVS) candidates from the $\mathit{\text{Gaia}}$ DR3 catalog with a $1\sigma$ lower limit of the tangential velocity of 800 km,s${}^{-1}$. J1903-0023, one of the brightest targets, stands out as high priority candidate for follow-up spectroscopy using the X-shooter instrument at ESO-VLT. We determine its atmospheric parameters and abundances utilizing synthetic spectral grids and a global ${\chi }^2-$minimization procedure, and its stellar parameters with the help of evolutionary tracks and the spectral energy distribution. The star shows variability in its light curve and follow-up spectroscopy confirms that the star is radial-velocity variable. The spectroscopic distance of J1903-0023 is lower than that based on the parallax, indicating that the star is not a hypervelocity binary star but bound to the Galaxy. The star turned out to be of spectral type F, very similar to the extreme-velocity star J0725-2351, which we analyse in the same way as the target. Apparently, both stars are very metal poor and old halo main-sequence (sdF) stars with masses slightly below the halo turn-off mass, and share the low metallicity ([Fe/H]=-2.3,-2.6) and strong alpha enhancement ([$\alpha$/Fe]$\sim 0.44$). While J0725-2351 is non-rotating ($v\sin i<3$,km,s${}^{-1}$), J1903-0023 is a fast rotator ($v\sin i=42.3\pm 2.0$ km,s${}^{-1}$). The Gaia, and ZTF light curves show an eclipse at a 1.179 day period, similar to the rotation period of J1903-0023. We therefore conclude that J1903-0023 is a high-velocity tidally-synchronised binary most likely with a metal-poor M dwarf companion.