Microlensing and Halo Mass in Form of Stars
  Paczy\'nski (1986) suggested that ``dark" objects in the halo of our Galaxy could enhance the luminosity of foreground stars, acting as gravitational microlenses. Such events has been recently reported by different collaborations. We assume that these microlensing events are produced by baryonic objects in the halo of our Galaxy. Rather than adopting a mean mass for the lensing objects we consider them distributed along a mass function similar to the disk one. Even if the number of microlensing events so far detected is not large, we show that it is possible to constrain the fraction of mass density in the halo of our Galaxy in form of stars. We estimate that this fraction varies between 0.1 and 0.8, if halo stars trace the total halo mass density; in the opposite case this range becomes narrower. If lensing objects have not been formed apart from the other halo stars, but in a more general context of star formation, neutron stars must also be present. We discuss these results in relation to galactic halo models for gamma-ray bursts. 
Sergio Campana (Osservatorio astronomico di Brera, Via Brera 28,   Milan, Italy)