  We present high-quality HST/GHRS spectra in the Hydrogen L alpha spectral
region of Vega and Sirius-A. Thanks to the signal-to-noise ratio achieved in
these observations and to the similarity of the two spectra, we found clear
evidence of emission features in the low flux region, 1190-1222 A. These
emission lines can be attributed unambiguously to Fe II and Cr II transitions.
In this spectral range, silicon lines are observed in absorption.
  We built a series of non-LTE model atmospheres with different, prescribed
temperature stratification in the upper atmosphere and treating Fe II with
various degrees of sophistication in non-LTE. Emission lines are produced by
the combined effect of the Schuster mechanism and radiative interlocking, and
can be explained without the presence of a chromosphere. Silicon absorption
lines and the L alpha profile set constraints on the presence of a
chromosphere, excluding a strong temperature rise in layers deeper than tau_R ~
10^{-4}.
