Hardness ratio diagrams for sources detected in the four new XMM-Newton observations of the northern disc of M 31. Only sources with hardness ratio errors EHR_i <  0.3 are shown. Different symbols are used for different source classes: red diamonds for foreground stars, orange squares for SNRs, cyan triangles for SSSs, light-blue crosses for X-ray binaries, and dark-blue Xs for background sources. Hard sources, which either can be AGNs or X-ray binaries, are marked with purple circles. Small symbols indicate candidates for each class, confirmed classifications are marked with large symbols. Black dots are used for unclassified sources. In HR₂–HR₃ diagram lines indicate the predicted position for sources with a power-law spectrum (dash-dotted lines) for Γ = 1 (black), 2 (red), 3 (blue) and a disc black-body spectrum (dashed-dot-dotted lines) for kT = 0.5 keV (black), 1.0 keV (light blue), 2.0 keV (green) for different absorbing foreground N_H (thin dotted for 10²⁰ cm⁻², dashed for 10²¹ cm⁻², and thick solid for 5 × 10²¹ cm⁻²).