Coadjoint orbits of Lie algebras and Cartan class

We study the coadjoint orbits of a Lie algebra in terms of Cartan class. In fact, the tangent space to a coadjoint orbit $mathcal{O}(alpha)$ at the point $alpha$ corresponds to the characteristic space associated to the left invariant form;$alpha$ and its dimension is the even part of the Cartan class of $alpha$. We apply this remark to determine Lie algebras such that all the nontrivial orbits (nonreduced to a point) have the same dimension, in particular when this dimension is 2 or 4. We determine also the Lie algebras of dimension $2n$ or $2n+1$ having an orbit of dimension $2n$.