  Point lens microlensing events with impact parameter close to the source
stellar radius allow the observer to study the surface brightness profile of
the lensed source. We have examined the effect of photospheric star spots on
multicolour microlensing lightcurves and investigated the detectability of such
spots in different wavebands as a function of spot temperature, position,
radius and lens trajectories. We include the effect of limb darkening and spot
projection as a function of position on the stellar disc. In particular we
apply the updated, state-of-the-art NEXTGEN stellar atmosphere models of
Hauschildt et al which predict very strong limb darkening and which are likely
to be applicable to the source stars considered here. Our results indicate that
star spots generally give a clear signature for transit events. Moreover, this
signature is strongly suppressed by limb darkening for spots close to the limb,
although the spots may still be clearly detected for favourable lens
trajectories. It is also clear that intensive temporal sampling throughout the
duration of the transit is necessary in order for such events to be effective
as a tool for imaging stellar photospheres. Nonetheless, with sufficiently well
sampled light curves of good photometric precision, microlensing can indeed
place useful constraints on the presence or otherwise of photospheric
starspots.
