  (abridged) The metal-rich bulge globular cluster NGC6388 shows a distinct
blue horizontal-branch tail with a strong upward tilt in its colour-magnitude
diagram. Several noncanonical scenarios have been proposed to explain these
puzzling observations. In order to test the predictions of these scenarios, we
have obtained medium resolution spectra to determine the atmospheric parameters
of a sample of the blue HB stars in NGC 6388. Using the medium resolution
spectra, we determine effective temperatures, surface gravities and helium
abundances by fitting the observed Balmer and helium lines with appropriate
theoretical stellar spectra. As we know the distance to the cluster, we can
verify our results by determining masses for the stars. During the data
reduction we took special care in subtracting the background, which is
dominated by the overlapping spectra of cool stars. The physical parameters of
the blue HB stars near 10,000 K support the helium pollution scenario. The low
gravities and masses found for the hot blue tail stars, however, are probably
caused by problems with the data reduction, most likely due to remaining
background light in the spectra, which would affect the fainter hot blue tail
stars much more strongly than the brighter cool blue tail stars. Our study of
the hot blue tail stars in NGC 6388 illustrates the obstacles which are
encountered when attempting to determine the atmospheric parameters of hot HB
stars in very crowded fields using ground-based observations. We discuss these
obstacles and offer possible solutions for future projects.
