Dichotomy for the Hausdorff dimension of nonergodic directions on translation surfaces

We study the ergodic properties of the translation surface $X_{\lambda ,\mu }$ formed by gluing two flat tori along a slit with holonomy $(\lambda ,\mu )\in {\mathbb{R}}^2$. Extending the dichotomy result of Cheung, Hubert, and Masur for the case $\mu =0$, we prove the following: for slits not parallel to any absolute homology class, the Hausdorff dimension of the set of nonergodic directions is either $0$ or $\frac{1}{2}$. This dichotomy is completely characterized by the Pérez-Marco condition expressed in terms of best approximation denominators. As a corollary, we obtain that the Pérez-Marco condition for best approximation denominators is norm-independent.