  Bipartite entangled quantum states with a positive partial transpose (PPT),
i.e., PPT entangled states, are usually considered very weakly entangled. Since
no pure entanglement can be distilled from them, they are also called bound
entangled. In this paper we present two classes of ($2d\times 2d$)-dimensional
PPT entangled states for any $d\ge 2$ which outperform all separable states in
metrology significantly. We present strong evidence that our states provide the
maximal metrological gain achievable by PPT states for a given system size.
When the dimension $d$ goes to infinity, the metrological gain of these states
becomes maximal and equals the metrological gain of a pair of maximally
entangled qubits. Thus, we argue that our states could be called "PPT
singlets."
