Ga-actions of fiber type on affine T-varieties

Understand

Let X be a normal affine T-variety, where T stands for the algebraic torus.

• We classify Ga-actions on X arising from homogeneous locally nilpotent derivations of fiber type.
• We deduce that any variety with trivial Makar-Limanov (ML) invariant is birationally decomposable as Y\times P^2, for some Y.
• Conversely, given a variety Y, there exists an affine variety X with trivial ML invariant birational to Y\times P^2.