Within the course of the present project, we have discovered so far
29 Ap stars with resolved magnetically split lines (16 of which are
reported for the first time in this paper). This raises the number
of such stars presently known to 42, compared to 12 when we started
this study (the 42nd star with magnetically resolved lines has been
discovered more recently by Babel et al. 1995). As of end of
August 1995, we had performed more than 750 measurements of the mean
magnetic field modulus of 40 of those 42 stars (compared to about
80 measurements in the literature at the time of the beginning of
our investigation). Those measurements, in the most favourable cases
(more than one third of the studied stars), reach an unprecedented
accuracy (
). Our study brought the number of stars with
field modulus measurements sampling (fairly) well the rotation
cycle from 4 to 16. For a few more stars good phase coverage should
soon be achieved. These achievements allowed us to discuss various
aspects of the magnetism of Ap stars in greater generality and on
sounder statistical grounds than had previously been possible.
As part of our search for resolved magnetically split spectral lines in Ap stars, we have, of course, achieved a large number of negative results. These are not without interest (in particular, they can still be used to derive constraints on the magnetic fields of the considered stars, as well as on their projected equatorial velocity). Their analysis will undoubtedly allow us to complement and to refine some of the results reported here, and we intend to devote a future publication to their presentation.
In the future, we aim at completing progressively the phase coverage
of the variations of the field modulus of the stars with resolved lines
for which the measurements presented in this paper sample only partly
the rotation period. Particular attention is paid to the very
slowly rotating stars, for which a good knowledge of the field may
prove essential for the understanding of the braking mechanisms that
must have been (and possibly still are) at work. Since the simultaneous
consideration of the variations of the field modulus and of the
longitudinal field is required to derive meaningful constraints
on the field geometrical structure, and since enough determinations
of 
are available at present for less than a dozen stars with
magnetically resolved lines, we have also undertaken a systematic
study of the longitudinal field of the other stars of that type.
Acknowledgements
This research has made use of the Simbad database, operated at CDS, Strasbourg, France. This work was done partly during a stay of SH at the European Southern Observatory (Santiago, Chile), as ESO Senior Visiting Scientist.