Period separation of mixed modes ΔT_obs as a function of the mean frequency separation of the pressure modes  ⟨ Δν ⟩ . Blue (purple) crosses represent the targets observed in the center (anticenter) direction. The dashed line indicates the frontier below which the frequency resolution is not fine enough for deriving ΔT_obs and the dotted line represents the location of the spurious signature that would correspond to the small separation δν₀₂ (Mosser et al. 2011). The two separate domains correspond to the location of ΔT₁ observed by Bedding et al. (2011) and theoretically expected (Montalbán et al., in prep.). The vertical thick line indicates the mean 1-σ error bars (the error bar on the large separation Δν is in fact very small).