Top: Herschel/HIFI spectra towards DR21(OH). The red line shows the [C i] line at 492 GHz, the blue line shows the [C i] line at 809 GHz and the black line the [C ii] line at 1.9 THz. The horizontal axis is the LSR velocity in km s^-1 and the vertical axis the antenna temperature in Kelvins. The continuum level for [C ii] corresponds to the SSB continuum level. Bottom: zoom on the [C i] lines (red,blue as above), and average [C ii] spectrum of the OFF positions (black). The continuum levels have been shifted for clarity in the bottom panel.