New upper limits on the interstellar O 2 abundance

We report new observations of molecular oxygen in absorption at z = 0.685 in front of the radio source B0218+357. The lines at 56.3 and 118.7 GHz have been observed, redshifted to 33.4 and 70.5 GHz respectively, with the 12m at Kitt Peak, 43m at Green Bank telescopes, and the 45m Nobeyama radio telescope. Deriving the surface filling factor of the absorbing dark cloud with other lines detected at nearby frequencies, we deduce from the upper limits on theO2 lines a relative abundance ofmolecular oxygenwith respect to carbonmonoxydeofO2/CO <∼ 2 10−3 at 1σ, seven times lower than the previous limit. The consequences of this result are discussed.