  Rather than "measuring" a scientist impact through the number of citations
which his/her published work can have generated, isn't it more appropriate to
consider his/her value through his/her scientific network performance
illustrated by his/her co-author role, thus focussing on his/her joint
publications, - and their impact through citations? Whence, on one hand, this
paper very briefly examines bibliometric laws, like the $h$-index and
subsequent debate about co-authorship effects, but on the other hand, proposes
a measure of collaborative work through a new index. Based on data about the
publication output of a specific research group, a new bibliometric law is
found.
  Let a co-author $C$ have written $J$ (joint) publications with one or several
colleagues. Rank all the co-authors of that individual according to their
number of joint publications, giving a rank $r$ to each co-author, starting
with $r=1$ for the most prolific.
  It is empirically found that a very simple relationship holds between the
number of joint publications $J$ by coauthors and their rank of importance,
i.e. $J \propto 1/r$. Thereafter, in the same spirit as for the Hirsch core,
one can define a "co-author core", and introduce indices operating on an
author. It is emphasized that the new index has a quite different
(philosophical) perspective that the $h$-index. In the present case, one
focusses on "relevant" persons rather than on "relevant" publications.
  Although the numerical discussion is based on one case, there is little doubt
that the law can be verified in many other situations. Therefore, variants and
generalizations could be later produced in order to quantify co-author roles,
in a temporary or long lasting stable team(s), and lead to criteria about
funding, career measurements or even induce career strategies.
