  We report extensive spectroscopic and differential photometric BVRI
observations of the active, detached, 1.309-day double-lined eclipsing binary
IM Vir, composed of a G7-type primary and a K7 secondary. With these
observations we derive accurate absolute masses and radii of M(1) = 0.981 +/-
0.012 M(Sun), M(2) = 0.6644 +/- 0.0048 M(Sun), R(1) = 1.061 +/- 0.016 R(Sun),
and R(2) = 0.681 +/- 0.013 R(Sun) for the primary and secondary, with relative
errors under 2%. The effective temperatures are 5570 +/- 100 K and 4250 +/- 130
K. The significant difference in mass makes this a favorable case for
comparison with stellar evolution theory. We find that both stars are larger
than the models predict, by 3.7% for the primary and 7.5% for the secondary, as
well as cooler than expected, by 100 K and 150 K, respectively. These
discrepancies are in line with previously reported differences in low-mass
stars, and are believed to be caused by chromospheric activity, which is not
accounted for in current models. The effect is not confined to low-mass stars:
the rapidly-rotating primary of IM Vir joins the growing list of objects of
near-solar mass (but still with convective envelopes) that show similar
anomalies. The comparison with the models suggests an age of 2.4 Gyr for the
system, and a metallicity [Fe/H] of approximately -0.3 that is consistent with
other indications, but requires confirmation.
