  HD188112 is an extremely low mass white dwarf in a close binary system.
According to a previous study, the mass of HD188112 is $\sim$0.24 Msun and a
lower limit of 0.73 Msun could be put for the mass of its unseen companion, a
compact degenarate object. We used HST STIS spectra to measure the rotational
broadening of UV metallic lines in HD188112, in order to put tighter
constraints on the mass of its companion. By assuming that the system in is
synchronous rotation, we derive a companion mass between 1.05 and 1.25 Msun. We
also measure abundances for magnesium, silicon, and iron, respectively log
$N$(X)/$N$(H) = $-$6.40, $-$7.25, and $-$5.81. The radial velocities measured
from the UV spectra are found to be in very good agreement with the prediction
based on the orbital parameters derived in the previous study made a decade
ago.
