  We report extensive photometric and spectroscopic observations of the 6.1-day
period, G+M-type detached double-lined eclipsing binary V530 Ori, an important
new benchmark system for testing stellar evolution models for low-mass stars.
We determine accurate masses and radii for the components with errors of 0.7%
and 1.3%, as follows: M(A) = 1.0038 +/- 0.0066 M(sun), M(B) = 0.5955 +/- 0.0022
M(sun), R(A) = 0.980 +/- 0.013 R(sun), and R(B) = 0.5873 +/- 0.0067 R(sun). The
effective temperatures are 5890 +/- 100 K (G1V) and 3880 +/- 120 K (M1V),
respectively. A detailed chemical analysis probing more than 20 elements in the
primary spectrum shows the system to have a slightly subsolar abundance, with
[Fe/H] = -0.12 +/- 0.08. A comparison with theory reveals that standard models
underpredict the radius and overpredict the temperature of the secondary, as
has been found previously for other M dwarfs. On the other hand, models from
the Dartmouth series incorporating magnetic fields are able to match the
observations of the secondary star at the same age as the primary (3 Gyr) with
a surface field strength of 2.1 +/- 0.4 kG when using a rotational dynamo
prescription, or 1.3 +/- 0.4 kG with a turbulent dynamo approach, not far from
our empirical estimate for this star of 0.83 +/- 0.65 kG. The observations are
most consistent with magnetic fields playing only a small role in changing the
global properties of the primary. The V530 Ori system thus provides an
important demonstration that recent advances in modeling appear to be on the
right track to explain the long-standing problem of radius inflation and
temperature suppression in low-mass stars.
