Notes. As discussed in the text, the uncertainties on the individual column densities are about 20%, while a conservative estimate of the uncertainties on the ratios between species are 30% from error propagation. Square brackets indicate tentative fits to marginal detections. ^(a) Column density of isotopologue relative to that of the main species given respectively without (N∕N_main) and with the statistical corrections (N_corr∕N_main). ^(b) Derived from the ¹⁸O-isotopologue assuming a ¹⁶O/¹⁸O abundance ratio of 560 (Wilson & Rood 1994). ^(c)Many lines of the main isotopologues of ketene are optically thick. The most reliable column density comes from the ¹³C isotopologues, but are consistent with a standard ¹²C/¹³C ratio for the few relatively thin transitions of the main isotopologues. ^(d) For the ¹³C isotopologue of dimethyl ether, the ratios with respect to the main isotopologue have been corrected by the statistical factor corresponding to the two indistinguishable carbon atoms. ^(e) Due to optical depth issues and fainter lines, the fits to formic acid rely on simultaneous fits of the main isotopologue and ¹³C isotopologue combined and the two deuterated variants, respectively (see discussion in Sect. A.8).