100″ × 100″ region of the field around 0540 marked with a box in Fig. 1. The images are obtained in the [O III]/0 (left) and [O III]/6000 (middle) bands with NTT/EMMI, and in the 1.5–6.4 keV X-ray range (right) with Chandra/ACIS (Hwang et al. 2001). The positions of all the slits used in the spectral observations, and of the detected H II regions are marked as in Fig. 1. The agreement between the optical and X-ray coordinates is subarcsecond since the pulsar was used as a reference (see Lundqvist et al. 2011). A 20″ × 20″ box marks the inner region of 0540 blown up in Fig. 4. We note that the H II structures named as “F1”, “F2”, “F4” and “F5” are projected on regions with strong X-ray emission from the SNR.