Observed lines with K_a = 0,1 of fulvenallene(c-C₅H₄CCH₂) towards TMC-1 (see line parameters in Table C.1). The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s⁻¹. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum derived for T_rot  =  10 K and N(c-C₅H₄CCH₂)  =  2.7 × 10¹² cm⁻². Blank channels correspond to negative features produced in the folding of the frequency switching data. Some K_a = 2,3 lines also appear in these panels (top left and fourth from bottom right). Additional K_a = 2,3 lines are shown in Fig. 3.