  We present a new long-duration parallax event from the OGLE-II database,
sc5_2859, which has the second longest time-scale ever identified (tE =
547.6{+22.6}{-7.8} days). We argue that both the lens and source reside in the
Galactic disk, making event sc5_2859 one of the first confirmed examples of
so-called disk-disk microlensing. We find that the source star is most probably
located at a distance of D_S ~ 2 kpc, and from this we conclude that the lens
is unlikely to be a main-sequence star due to the strict limits that can be
placed on the lens brightness. A simple likelihood analysis is carried out on
the lens mass, which indicates that the lens could be another candidate stellar
mass black hole. We recommend that spectroscopic observations of the source be
carried out in order to constrain the source distance, since this is the main
source of uncertainty in our analysis. In addition, we briefly discuss whether
there appears to be an excess of long duration microlensing events in the
OGLE-II catalogue.

Astrophysics