  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 timescales 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 sigma
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 the apparent age
spreads. We also find that the variability tends to move objects parallel to
isochrones in V/V-i' 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 PMS objects or arises as a result of longer term
variability associated with changes in accretion flow.
