Can variability account for apparent age spreads in OB association colour-magnitude diagrams?

We have investigated the role of photometric variability in causing the apparent age spreads observed in the colour–magnitude diagrams of OB associations. We have found that the combination of binarity, photometric uncertainty and variability on time-scales of a few years is not sufficient to explain the observed spread in either of the OB associations we have studied. Such effects can account for about half the observed spread in the σ Orionis subgroup and about 1/20 of the observed spread in Cep OB3b. This rules out variability caused by stellar rotation and rotation of structures within inner accretion discs as the source of the majority of the apparent age spreads. We also find that the variability tends to move objects parallel to isochrones in V/V − i � colour–magnitude diagrams (CMDs), and thus has little influence on apparent age spreads. We conclude that the remaining unexplained spread either reflects a true spread in the ages of the pre-main-sequence (PMS) objects or arises as a result of longer term variability associated with changes in accretion flow.