  On the basis of the best available member list and duplicity information, we
have studied the radial distribution of 270 stars and multiple systems earlier
than K0 in the Pleiades. Five new long period spectroscopic binaries have been
identified from the CORAVEL observations. We have found a clear mass
segregation between binaries and single stars, which is explained by the
greater average mass of the multiple systems. The mass function of the single
stars and primaries appears to be significantly different. While the central
part of the cluster is spherical, the outer part is clearly elliptical, with an
ellipticity of 0.17. The various parameters describing the Pleiades are (for a
distance of 125 pc): core radius r_{c}=0.6 deg (1.4 pc), tidal radius r_{t}=7.4
deg (16 pc), half mass radius r_{m/2}=0.88 deg (1.9 pc), harmonic radius r=1.82
deg (4 pc). Low-mass stars (later than K0) probably extend further out and new
proper motion and radial velocity surveys over a larger area and to fainter
magnitudes would be very important to improve the description of the cluster
structure and complete mass function.
