Comparison of the relative radial velocities between the HC₃N (2–1) and NH₃ (1, 1) lines measured at the same coordinates in Feb. 2009 (filled circles) and in Jan. 2010 (open circles). The 2009 data are corrected by 4 m s^-1 as described in Sect. 1. For L1517B the offset (Δα,Δδ)₂₀₁₀ = (15′′,15′′) corresponds to the offset (Δα,Δδ)₂₀₀₉ = (0′′,0′′). The mean value is depicted for multiple observations at the same coordinate. The error bars show the 1σ uncertainties.