  The chemical composition of stellar photospheres in mass-transferring binary
systems is a precious diagnostic of the nucleosynthesis processes that occur
deep within stars, and preserves information on the components history. The
binary system u Her belongs to a group of hot Algols with both components being
B-stars. We have isolated the individual spectra of the two components by the
technique of spectral disentangling of a new series of 43 high-resolution
echelle spectra. Augmenting these with an analysis of the Hipparcos photometry
of the system yields revised stellar quantities for the components of u Her.
For the primary component (the mass-gaining star) we find M_A = 7.88 +/- 0.26
M_sun, R_A = 4.93 +/- 0.15 R_sun and T_eff_A = 21600 +/- 220 K. For the
secondary (the mass-losing star) we find M_B = 2.79 +/- 0.12 M_sun, R_B = 4.26
+/- 0.06 R_sun and T_eff_B = 12600 +/- 550 K. A non-LTE analysis of the primary
star's atmosphere reveals deviations in the abundances of nitrogen and carbon
from the standard cosmic abundance pattern in accord with theoretical
expectations for CNO nucleosynthesis processing. From a grid of calculated
evolutionary models the best match to the observed properties of the stars in u
Her enabled tracing the initial properties and history of this binary system.
We confirm that it has evolved according to case A mass transfer. A detailed
abundance analysis of the primary star gives C/N = 0.9, which supports the
evolutionary calculations and indicates strong mixing in the early evolution of
the secondary component, which was originally the more massive of the two. The
composition of the secondary component would be a further important constraint
on the initial properties of u Her system, but requires spectra of a higher
signal to noise ratio.
