The HCO radical was detected in the ISM a long time ago. The detection of the HCCO radical, however, was found only fairly recently.
B. A. McGuire, A. M. Burkhardt, C. N. Shingledecker, S. V. Kalenskii, E. Herbst, A. J. Remijan, and M. C. McCarthy
reported on the
Detection of Interstellar HC₅O in TMC-1 with the Green Bank Telescope
Astrophys. J. 843, Art. No. L28 (2017).
Two Λ-components of butadiynylformyl, each consisting of two HFS components, of the J = 17/2 – 15/2 transition were detected in emission near 21.945 GHz.
In addition, the hexatriynylformyl radical, HC₇O, was tentatively detected through stacking the Λ-components of two transitions, J = 35/2 – 33/2 and 37/2 – 35/2 near 19.204 and 20.302 GHz, respectively. These reports may appear as space oddities, but we shall know more eventually.

M. A. Cordiner, S. B. Charnley, Z. Kisiel, B. A. McGuire, and Y.-J. Kuan
reported on the
Deep K-band Observations of TMC-1 with the Green Bank Telescope: Detection of HC₇O, Nondetection of HC₁₁N, and a Search for New Organic Molecules
Astrophys. J. 850, Art. No. 187 (2017).
The Λ-components of two hexatriynylformyl transitions, J = 35/2 – 33/2 near 19.204 GHz (as before) and 33/2 – 31/2 near 18.107 GHz were detected weakly. Stacking of these data together with those from above yielded a reasonable detection.
See also HCCCO contribution.

J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, N. Marcelino, R. Fuentetaja, J. R. Pardo, and P. de Vicente
reported on the
Discovery of HCCCO and C₅O in TMC-1 with the QUIJOTE Line Survey
Astron. Astrophys. 656, Art. No. L21 (2021).
Propynonyl, HC₃O, was identified through two rotational transitions with 2 ≤ J ≤ 4 in the course of a molecular line survey of the prototypical cold dark molecular cloud TMC-1 carried out with the Yebes 40 m radio telescope between 31.0 and 50.3 GHz. 7 of the 8 strong hyperfine components of four fine structure components were detected clearly; two HFS components were unresolved. A value of T_rot = 7 K was taken from C₃O. HC₅O is the most abundant of the HC_nO molecules with n ≥ 1. Setting its relative abundance to 100, they obtained for HCO, HC₂O, HC₃O, HC₅O, and HC₇O the approximate ratios 79 : 55 : 9 : 100 : 46. Only upper limits were derived for HC₄O and HC₆O; these were in both cases similar to those of HC₃O and are thus somewhat constraining.

Contributor(s): H. S. P. Müller; 07, 2017; 12, 2017; 12, 2021