(a) PV diagram of ¹²CO (1–0) along the crest of F1 as indicated in Fig. 3f. The overlaid contours represent the PV diagram of ¹³CO (1–0). The contours start at 0.9 K (3σ) and increase by 0.9 K. The offsets are given with respect to the southwestern end of the PV cut. The systemic velocities of L1188a and L1188b are indicated by two blue vertical dashed lines. (b) Similar to a, but for ¹²CO (2–1). (c) Similar to a, but for ¹²CO (3–2). In panels b–c, the bridging features are indicated by the green dashed boxes. (d) Normalized integrated intensities of ¹²CO (2–1) (blue points) and ¹²CO (3–2) (red points) as a function of the offsets in the crest of F1. The offsets are calculated with respect to the southwestern end of F1.