  We present the analysis of the microlensing event OGLE-2015-BLG-0845, which
was affected by both the microlensing parallax and xallarap effects. The former
was detected via the simultaneous observations from the ground and Spitzer, and
the latter was caused by the orbital motion of the source star in a relatively
close binary. The combination of these two effects led to a mass measurement of
the lens object, revealing a low-mass ($0.14 \pm 0.05 M_{\odot}$) M-dwarf at
the bulge distance ($7.6 \pm 1.0$ kpc). The source binary consists of a late
F-type subgiant and a K-type dwarf of $\sim1.2 M_{\odot}$ and $\sim 0.9
M_{\odot}$, respectively, and the orbital period is $70 \pm 10$ days.
OGLE-2015-BLG-0845 is the first single-lens event in which the lens mass is
measured via the binarity of the source. Given the abundance of binary systems
as potential microlensing sources, the xallarap effect may not be a rare
phenomenon. Our work thus highlights the application of the xallarap effect in
the mass determination of microlenses, and the same method can be used to
identify isolated dark lenses.
