Context : Astrometric microlensing is an excellent tool to determine the mass of stellar objects .
By measuring the astrometric shift of a background source star in combination with precise predictions of its unlensed position and of the lens position , gravitational lensing allows to one determine the mass of the lensing star with a precision of 1 percent , independently of any prior knowledge .

Aims : Making use of the recently published Gaia Data Release 2 ( Gaia DR2 ) we predict astrometric microlensing events by foreground stars of high proper motion passing by a background star in the coming years .

Methods : We compile a list of approximately 148,000 high-proper-motion stars within Gaia DR2 with \mu _ { tot } > 150 \mathrm { mas / yr } .
We then search for background stars close to their paths and calculate the dates and separations of the closest approaches .
Using color and absolute magnitude , we determine approximate masses of the lenses .
Finally , we calculate the expected astrometric shifts and magnifications of the predicted events .

Results : We detect two ongoing microlensing events by the high-proper-motion stars Luyten 143-23 and Ross 322 and predict closest separations of ( 108.5 \pm 1.4 ) \mathrm { mas } in July 2018 and ( 125.3 \pm 3.4 ) \mathrm { mas } in August 2018 , respectively .
The respective expected astrometric shifts are ( 1.74 \pm 0.12 ) \mathrm { mas } and ( 0.76 \pm 0.06 ) \mathrm { mas } .
Furthermore , Luyten 143-23 will pass by another star in March 2021 with a closest separation of ( 280.1 \pm 1.1 ) \mathrm { mas } , which results in an expected shift of ( 0.69 \pm 0.05 ) \mathrm { mas } .

Conclusions :