An upper limit to the hydroxyl abundance in the circumstellar envelope of IRC + 10216

We have searched for thermal OH emission from IRC + 10216 in the 1665- and 1667-MHz lines. No emission was detected, yielding an upper limit for the OH abundance in the envelope. Within the photodissociation region where OH is expected to occur, $$\text{[OH]}/\text{[CO]}\,\lt\,2.7\,\times\,{10}^{-3}$$.