  The activation gap $\Delta$ of the fractional quantum Hall states at constant
fillings $\nu =2/3$ and 2/5 has been measured as a function of the
perpendicular magnetic field $B$. A linear dependence of $\Delta$ on $B$ is
observed while approaching the spin polarization transition. This feature
allows a direct measurement of the $g$-factor of composite fermions which
appears to be heavily renormalized by interactions and strongly sensitive to
the electronic filling factor.
