  This paper examines the relationship between spectra of stars of same
spectral type with extremely low reddenings.
  According to the standard theory, the relationship between the spectrum of
stars with same spectral type and small, but different reddenings should be
different in the optical and in the UV.
  This difference is not observed: the ratio of the spectra of two stars in
directions where the reddening is large enough to be detected and low enough
not to give a noticeable 2200Ang. bump is an exponential of 1/lambda from the
near-infrared to the far-UV.
  This result is in conformity with the ideas introduced in preceding papers:
the exponential optical extinction extends to the UV, and the spectrum of stars
with enough reddening is contaminated by light scattered at close angular
distance from the stars.
  An application will be the determination of the spectrum of a non-reddened
star from the spectrum of a star of same spectral type with little reddening.
