  We present the results of spectroscopic analyses of hot horizontal branch
(HB) stars in M 13 and M 3, which form a famous ``second parameter'' pair. From
the spectra and Stroemgren photometry we derived -- for the first time in M 13
-- atmospheric parameters (effective temperature and surface gravity). For
stars with Stroemgren temperatures between 10,000 and 12,000 K we found
excellent agreement between the atmospheric parameters derived from Stroemgren
photometry and those derived from Balmer line profile fits. However, for cooler
stars there is a disagreement in the parameters derived by the two methods, for
which we have no satisfactory explanation. Stars hotter than 12,000 K show
evidence for helium depletion and iron enrichment, both in M 3 and M 13.
Accounting for the iron enrichment substantially improves the agreement with
canonical evolutionary models, although the derived gravities and masses are
still somewhat too low. This remaining discrepancy may be an indication that
scaled-solar metal-rich model atmospheres do not adequately represent the
highly non-solar abundance ratios found in blue HB stars affected by diffusion.
We discuss the effects of an enhancement in the envelope helium abundance on
the atmospheric parameters of the blue HB stars, as might be caused by deep
mixing on the red giant branch or primordial pollution from an earlier
generation of intermediate mass asymptotic giant branch stars.

Astrophysics