  We present new V-band differential photometry and radial-velocity
measurements of the unevolved 1.18-day period F+G-type double-lined eclipsing
binary VZ Cep. We determine accurate values for the absolute masses, radii, and
effective temperatures as follows: M(A) = 1.402 +/- 0.015 M(Sun), R(A) = 1.534
+/- 0.012 R(Sun), T(eff) = 6690 +/- 160 K for the primary, and M(B) = 1.1077
+/- 0.0083 M(Sun), R(B) = 1.042 +/- 0.039 R(Sun), T(eff) = 5720 +/- 120 K for
the secondary. A comparison with current stellar evolution models suggests an
age of 1.4 Gyr for a metallicity near solar. The temperature difference between
the stars, which is much better determined than the absolute values, is found
to be about 250 K larger than predicted by theory. If all of this discrepancy
is attributed to the secondary (which would then be too cool compared to
models), the effect would be consistent with similar differences found for
other low-mass stars, generally believed to be associated with chromospheric
activity. However, the radius of VZ Cep B (which unlike the primary, still has
a thin convective envelope) appears normal, whereas in other stars affected by
activity the radius is systematically larger than predicted. Thus, VZ Cep poses
a challenge not only to standard theory but to our understanding of the
discrepancies in other low-mass systems as well.
