SIMBAD references

We present new K - and L'-band imaging of a representative sample of members of the young 3-5 Myr old σ Orionis cluster. We identified objects with (K -L') excess by analysing colour-colour diagrams and comparing the observations with empirical main-sequence colours. The derived disc frequency depends on the method used: (54±15) per cent if measured directly from the JHKL' colour-colour diagram, or (46±14) per cent if excesses are computed with respect to predicted photospheric colours (according to the object spectral types, 2σ excess detections). We compare the (K-L') excess with other indicators and show that this is a robust and reliable disc indicator. We also compare the derived disc frequency with similarly aged clusters and discuss possible implications for disc lifetimes. The computed age of the σ Ori cluster is very important; a cluster age of 3 Myr would support the overall disc lifetime of 6 Myr proposed in the literature, while an age >4 Myr would point to a slower disc destruction rate.