Spectral energy distribution (top panel), ionisation balance (middle panel), and stability curve (bottom panel). The band covered by XMM-Newton is shown in grey. Combined+V refers to the multi-wavelength SED built with XMM-Newton X-ray data and HST optical data. The ionisation balance and stability curves of the individual observations are not shown since they are almost superimposable. The best-fit ranges of the emission (pion) and absorption (xabs) line components obtained for the individual observations are shown as thicker segments (see Table 1).