  We study the interaction-enhanced spin gaps in the two-dimensional electron
gas confined in GaAs/AlGaAs single heterojunctions subjected to weak magnetic
fields. The values are obtained from the chemical potential jumps measured by
magnetocapacitance. The gap increase with parallel magnetic field indicates
that the lowest-lying charged excitations are accompanied with a single spin
flip at the odd-integer filling factor nu=1 and nu=3, in disagreement with the
concept of skyrmions.
