What set is $\mathbb{T}^1$?

I am reading Farnocchia et al "Robust resolution of Kepler’s equation in all eccentricity regimes" and found this line:

For eccentricity-true anomaly variables $$(e, f) \in \mathbb{R}^+ \times \mathbb{T}^1$$, by excluding the physically unallowed regions where $$1 + e \cos f \le 0$$, and for a small positive $$\delta > 0$$, we split the set $$\Omega = \{ (e, f) \in \mathbb{R}^+ \times \mathbb{T}^1 ; 1 + e \cos f > 0\ \}$$ into three sets: ...

The eccentricity $$e$$ is a positive real number (so far so good), but the true anomaly $$f$$, which is supposed to be an angle, is said to belong to this set $$\mathbb{T}^1$$ which I have never seen.

It's not defined anywhere in the paper (so I guess it should be super obvious), it's not mentioned in any of the references, and I can't seem to find it in Wikipedia or MathWorld. Does anybody have a clue of what this can mean?

Use of "$$\cos f$$" suggests this circle is parametrized by angle, $$[0, 2\pi]$$ or some other $$2\pi$$-long interval.