Interstellar reddening towards six small areas in Puppis-Vela
  We investigate the distribution of the interstellar dust towards six small
volumes of the sky in the region of the Gum nebula. New high-quality
four-colour uvby and H\beta\ Str\"omgren photometry obtained for 352 stars in
six selected areas of Kapteyn, complemented with data obtained in a previous
investigation for two of these areas, were used to estimate the colour excess
and distance to these objects. The obtained colour excess versus distance
diagrams, complemented with other information, when available, were analysed in
order to infer the properties of the interstellar medium permeating the
observed volumes. On the basis of the overall standard deviation in the
photometric measurements, we estimate that colour excesses and distances are
determined with an accuracy of 0.010 mag and better than 30%, respectively, for
a sample of 520 stars. A comparison with 37 stars in common with the new
Hipparcos catalogue attests to the high quality of the photometric distance
determination. The obtained colour excess versus distance diagrams testify to
the low density volume towards the observed lines-of-sight. Very few stars out
to distances of 1 kpc from the Sun have colour excesses larger than E(b-y) =
0.1 mag. In spite of the low density character of the interstellar medium
towards the Puppis-Vela direction, the obtained reddening as a function of the
distance indicates that two or more interstellar structures are crossed towards
the observed lines-of-sight. One of these structures may be associated with the
very low density wall of the Local Cavity, which has a distance of 100-150 pc
from the Sun. Another structure might be related to the Gum nebula, and if so,
its front face would be located at about 350 pc from the Sun.
