SIMBAD references

We have selected pre-main-sequence (PMS) stars in 12 groups of notional ages ranging from 1 to 35 Myr, using heterogeneous membership criteria. Using these members we have constructed empirical isochrones in V, V - I colour-magnitude diagrams. This allows us to identify clearly the gap between the radiative main sequence and the convective PMS (the R-C gap). We follow the evolution of this gap with age and show that it can be a useful age indicator for groups less than ≃15 Myr old. We also observe a reduction in absolute spreads about the sequences with age. Finally, the empirical isochrones allow us to place the groups in order of age, independently of theory. The youngest groups can be collated into three sets of similar ages. The youngest set is the ONC, NGC6530 and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori and NGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally 4-5 Myr). This suggests Cep OB3b is younger than previously thought, and IC348 older. For IC348 the stellar rotation rate distribution and fraction of stars with discs imply a younger age than we derive. We suggest this is because of the absence of O-stars in this cluster, whose winds and/or ionizing radiation may be an important factor in the removal of discs in other clusters.