  Previous calculations of the rates and optical depths due to microlensing
only considered resolved stars. However, if a faint unresolved star lens is
close enough to a resolved star, the event will be seen by the microlensing
experiments and attributed to the bighter star. The blending biases the
duration, making the contribution of the unresolved stars very significant for
short events. This contribution is confused with lensing by brown dwarfs. The
exact rates of these blended events are extremly sensitive to the limiting
magnitude achieved in the microlensing search. Appropriate calculations of the
optical depth and rates are provided here, and illustrated in the case of the
DUO and OGLE experiments. The additional contribution of unresolved stars is
very significant and probably explains the high optical depth and rates
observed towards the Galactic Bulge. The blended unresolved event can be
identified using either the color shift or the light curve shape. However,
neither of these two methods is apropriate to identify a large number of
blended events towards the Bulge. In some cases of good photometry and small
impact parameter, an identification is possible, as for the OGLE 5 event, which
clearly appears as a case of lensing of an unresolved star. The recent results
obtained by the PLANET collaboration indicate that a high resolution and dense
sampling of the light curve is possible, and will probably provide a very
interesting possibility to correct the blending bias, as demonstrated for OGLE
5. This possibility, is certainly better than a statistical estimation of the
lensing rates, which are always prone to some uncertainty. But, at this time,
the contribution of unresolved stars must be included in the analyses of
microlensing experiments.
