  We present the analysis of the event OGLE-2017-BLG-1186 from the 2017 Spitzer
microlensing campaign. This is a remarkable microlensing event because its
source is photometrically bright and variable, which makes it possible to
perform an asteroseismic analysis using ground-based data. We find that the
source star is an oscillating red giant with average timescale of $\sim 9$ d.
The asteroseismic analysis also provides us source properties including the
source angular size ($\sim 27~\mu{\rm as}$) and distance ($\sim 11.5$ kpc),
which are essential for inferring the properties of the lens. When fitting the
light curve, we test the feasibility of Gaussian Processes (GPs) in handling
the correlated noise caused by the variable source. We find that the parameters
from the GP model are generally more loosely constrained than those from the
traditional $\chi^2$ minimization method. We note that this event is the first
microlensing system for which asteroseismology and GPs have been used to
account for the variable source. With both finite-source effect and microlens
parallax measured, we find that the lens is likely a $\sim 0.045~M_{\odot}$
brown dwarf at distance $\sim 9.0$ kpc, or a $\sim 0.073~M_{\odot}$ ultracool
dwarf at distance $\sim 9.8$ kpc. Combining the estimated lens properties with
a Bayesian analysis using a Galactic model, we find a $\sim 35$ per cent
probability for the lens to be a bulge object and $\sim 65$ per cent to be a
background disc object.
