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 \pm 0.015 M _ { \sun } , R _ { A } = 1.534 \pm 0.012 R _ { \sun } , T _ { eff } = 6690 \pm 160 K for the primary , and M _ { B } = 1.1077 \pm 0.0083 M _ { \sun } , R _ { B } = 1.042 \pm 0.039 R _ { \sun } , T _ { eff } = 5720 \pm 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 \sim 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 .