  We analyze PLANET photometric observations of the caustic-crossing
binary-lens microlensing event, EROS BLG-2000-5, and find that modeling the
observed light curve requires incorporation of the microlens parallax and the
binary orbital motion. The projected Einstein radius (~r_E = 3.61 +/- 0.11 AU)
is derived from the measurement of the microlens parallax, and we are also able
to infer the angular Einstein radius (theta_E = 1.38 +/- 0.12 mas) from the
finite source effect on the light curve, combined with an estimate of the
angular size of the source given by the source position in a color-magnitude
diagram. The lens mass, M = 0.612 +/- 0.057 M_sun, is found by combining these
two quantities. This is the first time that parallax effects are detected for a
caustic-crossing event and also the first time that the lens mass degeneracy
has been completely broken through photometric monitoring alone. The
combination of the projected Einstein radius and the angular Einstein radius
also allows us to conclude that the lens lies in the near side of the disk,
within 2.6 kpc of the Sun, while the radial velocity measurement indicates that
the source is a Galactic bulge giant.

Astrophysics