  We study polynomial systems whose equations have as common support a set C of
n+2 points in Z^n called a circuit. We find a bound on the number of real
solutions to such systems which depends on n, the dimension of the affine span
of the minimal affinely dependent subset of C, and the "rank modulo 2" of C. We
prove that this bound is sharp by drawing so-called dessins d'enfant on the
Riemann sphere. We also obtain that the maximal number of solutions with
positive coordinates to systems supported on circuits in Z^n is n+1, which is
very small comparatively to the bound given by the Khovanskii fewnomial
theorem.
