Solar twins in M67

The discovery of true solar analogues is fundamental for a better understanding of the Sun and of the solar system. The open cluster M67 offers a unique opportunity to search for solar analogues because its chemical composition and age are very similar to those of the Sun. We analyze FLAMES spectra of a large number of M67 main sequence stars to identify solar analogues in this cluster.We first determine cluster members which are likely not binaries, by combining proper motions and radial velocity measurements. We concentrate our analysis on the determination of stellar effective temperature, using analyses of line-depth ratios and H$alpha$ wings, making a direct comparison with the solar spectrum obtained with the same instrument. We also compute the lithium abundance for all the stars.Ten stars have both the temperature derived by line-depth ratios and H$alpha$ wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening $E(B-V)=0.041$, the solar colour $(B-V)_odot=0.649pm0.016$ and a cluster distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun and candidates to be true solar twins. These stars have also a low Li content, comparable to the photospheric abundance of the Sun, likely indicating a similar mixing evolution. We find several candidates for the best solar analogues ever. These stars are amenable to further spectroscopic investigations and planet search. The solar colours are determined with rather high accuracy with an independent method, as well as the cluster distance modulus.