Frequency shifts of the l = 0 (blue squares) and l = 1 modes (red diamonds) of HD 49933 as a function of frequency obtained from method #1. The corresponding weighted mean values of the frequency shift between consecutive l = 0 and 1 modes are represented by the black dots (solid line). A zoom-in on the frequency range up to 2100 μHz is shown in the insert, which represents the mean frequency shift. The horizontal blue dashed lines correspond to the frequency shifts obtained with the cross-correlation method (#3). The solid red line corresponds to a weighted linear fit.