Detection of a new linear carbon chain radical: C_7_H.

Following our discovery of C_8_H in IRC+10216 (Cernicharo & Guelin 1996A&A...309L..27C), we report the detection in this circumstellar envelope of another linear carbon chain radical, C_7_H. The microwave spectrum of C_7_H has been recently observed in the laboratory (Travers et al. 1996ApJ...465L..77T) and its rotational line frequencies are precisely known. With this new detection, the family of acetylenic chain radicals (C_n_H) observed in space is complete up to n=8. The members with even numbers of carbon atoms are consistently more abundant than the odd number members; C_7_H is found to be a factor of 4 less abundant than C_8_H and a factor of 20 less abundant than C_5_H.