We consider small perturbations periodic in time of an oscillator whose restoring force has a leading term with exponent 3 or 1/3. The first case corresponds to oscillations with infinitesimal frequency and the second case to oscillations with infinite frequency. The smallness of the perturbation is determined both by the smallness of the considered neighborhood of the equilibrium point and by a small nonnegative parameter ε. For ε = 0, the stability of the equilibrium point is studied. For ε > 0, we find conditions for an invariant two-dimensional torus to branch off with "soft" or "rigid" loss of stability with loss index 1/2.