Each co-author (CA) of any scientist can be given a rank (r) of importance according to the number (J) of joint publications which the authors have together. In this paper, the Zipf-Mandelbrot-Pareto law, i.e. J ∝ 1/(ν + r) ζ is shown to reproduce the empirical relationship between J and r and shown to be preferable to a mere power law, J ∝ 1/r α . The CA core value, i.e. the core number of CAs, is unaffected, of course. The demonstration is made on data for two authors, with a high number of joint publications, recently considered by Bougrine (2014) and for 7 authors, distinguishing between their "journal" and "proceedings" publications as suggested by Miskiewicz (2013). The rank-size statistics is discussed and the α and ζ exponents are compared. The correlation coefficient is much improved (∼ 0.99, instead of 0.92). There are marked deviations of such a co-authorship popularity law depending on sub-fields. On one hand, this suggests an interpretation of the parameter ν. On the other hand, it suggests a novel model on the (likely time dependent) structural and publishing properties of research teams.Thus, one can propose a scenario for how a research team is formed and grows. This is based on a hierarchy utility concept, justifying the empirical Zipf-Mandelbrot-Pareto law, assuming a simple form for the CA publication/cost ratio, cr = c0 log2(ν + r). In conclusion, such a law and model can suggest practical applications on measures of research teams.In Appendices, the frequency-size cumulative distribution function is discussed for two sub-fields, with other technicalities * previously at GRAPES, ULG, Liège, Belgium;