  We present a detailed analysis of the UV spectrum of the calibration star
Bd+28 4211 using high-quality spectra obtained with the HST and FUSE
satellites. To this aim, we compare quantitatively the observed data with model
spectra obtained from state-of-the-art NLTE metal line-blanketed model
atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC. We thus
determine in a self-consistent way the abundances of eleven elements with
well-defined lines in the UV, namely those of C, N, O, F, Mg, Si, P, S, Ar, Fe,
and Ni. The derived abundances range from about solar to 1/10 solar. We find
that the overall quality of the derived spectral fits is very satisfying. Our
spectral analysis can be used to constrain rather tigthly the effective
temperature of Bd+28 to a value of teff = 82,000 +/- 5000 K. We also estimate
conservatively that its surface gravity falls in the range log g = 6.2
-0.1/+0.3. Assuming that the Hipparcos measurement for Bd+28 is fully reliable
and that our model atmospheres are reasonably realistic, we can reconcile our
spectroscopic constraints with the available parallax measurement only if the
mass of Bd+28 is significantly less than the canonical value of 0.5 msun for a
representative post-EHB star.
