Real and complex rank for real symmetric tensors with low complex symmetric rank

We study the case of real homogeneous polynomial $P$ whose minimal real and complex decompositions in terms of powers of linear forms are different. In particularly we will show that, if the sum of the complex and the real ranks of $P$ is smaller or equal than $ 3\deg(P)-1$, then the difference of the two decompositions is completely determined either on a line or on a conic.