Measuring the magnetization and the specific heat of La La_1-xSr_xMnO₃ single crystals with 0.11≤x≤0.15 we have investigated the charge order (CO) transition in lightly doped manganites. Though the properties of the ferromagnetic insulating state at low temperatures hardly depend on x, there is a very strong concentration dependence of the specific heat and the magnetization. While the specific-heat anomaly due to ferromagnetic order strongly increases with x, the jumps of both, entropy and magnetization at the CO transition decreases drastically. The analysis of our data shows that the entropy changes at the CO transition are mainly due to spin degrees of freedom, which strongly suggests that the melting of charge order is driven by magnetic energy.