The very weak a1Δg−X3Σg− system of the three 17 O isotopologues of oxygen – 16 O 17 O, 17 O 18 O and 17 O 2 – was studied by high sensitivity CW-Cavity Ring Down Spectroscopy. The spectra of a 17 O highly enriched sample were recorded at room temperature between 7640 and 7917cm −1 and at liquid nitrogen temperature in the 7876–7893cm −1 region. The magnetic dipole (0–0) band was observed for all three 17 O isotopologues. At liquid nitrogen temperature, some of the transitions were observed with partially resolved hyperfine splitting due to the 17 O nuclear spin. The electric quadrupole (0–0) band and the (1–1) hot band were also observed for the 16 O 17 O and 17 O 2 species. The rotational and hyperfine spectroscopic parameters of the X3Σg− and a 1 Δ g states of the three studied isotopologues were derived from a global fit of the measured line positions and microwave and Raman measurements available in the literature. The spectroscopic constants of the a 1 Δ g (v=0, 1) states of 17 O 2 are reported for the first time.