**Table 1:** Number in parentheses are 1 $\sigma$ uncertainties in units of the last digits.
**Observed C**₄H^- **and C**₆H^- **line parameters**
Transition	$\nu_{\rm cal}$	$\nu_{\rm obs}$	$\int$ $T_{\rm A}^*$ dv	$v_{\rm exp}$
$J'\rightarrow J''$	(MHz)	(MHz)	(K km s^-1)	(km s^-1)
		C₄H^-
9-8	83787.299	83787.4(10)	0.19(3)^a	14.5^b
11-10	102405.664	102405.7(8)	0.25(2)	14.4(5)
12-11	111714.621	111714.2(10)	0.17(3)^a	14.5(8)
14-13	130332.013	130332.9(8)	0.30(4)	14.8(8)
15-14	139640.420	139640.6(8)	0.16(3)	14.7(7)
		C₆H^-
29-28	79854.897	79854.9(3)	0.59(4)	13.5(6)
30-29	82608.285	82608.4(2)	0.63(2)	14.2(3)
32-31	88114.991	88114.9(2)	0.59(2)	14.4(2)
33-32	90868.306	90868.2(6)	0.41(4)	14.5^b
34-33	93621.597	93621.2(5)	0.31(3)	14.5^b
35-34	96374.861	96374.5(6)	0.32(3)	14.5^b
36-35	99128.097	99128.0(10)	0.23(2)^a	14.5^b
37-36	101881.306	101880.6(10)	0.25(2)^a	14.5^b
38-37	104634.486	104634.2(8)	0.30(3)	14.5^b
40-39	110140.757	110140.5(1)	0.20(3)	13.7(7)
41-40	112893.846	112893.9(8)	0.15(3)	14.5^b
42-41	115646.903	115645.9(10)	0.22(4)	14.5^b

^a denotes a partly blended line and ^b indicates that terminal
velocity has been fixed to 14.5 km s^-1.

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