  In the past, SDSS J160429.12+100002.2 was spectroscopically classified as a
blue horizontal branch (BHB) star. Assuming a luminosity that is characteristic
of BHB stars, the object's radial velocity and proper motions from Gaia Early
Data Release 3 would imply that its Galactic rest-frame velocity exceeds its
local escape velocity. Consequently, the object would be considered a
hypervelocity star, which would prove particularly interesting because its
Galactic trajectory points in our direction. However, based on the
spectroscopic analysis of follow-up observations, we show that the object is
actually a short-period ($P \approx 3.4$ h) single-lined spectroscopic binary
system with a visible B-type star (effective temperature $T_{\mathrm{eff}} =
15\,840\pm160$ K and surface gravity $\log(g) = 4.86\pm0.04$) that is less
luminous than typical BHB stars. Accordingly, the distance of the system is
lower than originally thought, which renders its Galactic orbit bound to the
Galaxy. Nevertheless, it is still an extreme halo object on a highly retrograde
orbit. The abundances of He, C, N, O, Ne, Mg, Al, Si, S, and Ca are subsolar by
factors from 3 to more than 100, while Fe is enriched by a factor of about 6.
This peculiar chemical composition pattern is most likely caused by atomic
diffusion processes. Combining constraints from astrometry, orbital motion,
photometry, and spectroscopy, we conclude that the visible component is an
unevolved proto-helium white dwarf with a thin hydrogen envelope that was
stripped by a substellar companion through common-envelope ejection. Its unique
configuration renders the binary system an interesting test bed for stellar
binary evolution in general and common-envelope evolution in particular.
