Continuing the theme of S-containing species, the syn (or cis) form of O-protonated sulfur dioxide (syn-HOSO⁺) is a molecule that may plausibly be present in space. Its rotational spectrum was measured by Lattanzi and co-workers, as reported in 2011. The syn (cis) form of HOSO⁺ is marginally more stable than the anti (trans) form. A proton can also be added to the sulfur atom of SO₂ to form S-protonated HSO₂⁺, but it lies much higher in energy than either O-protonated form. The parent molecule, sulfur dioxide, was first detected over 50 years ago.