We present new spectroscopic observations of the detached late-type double-lined eclipsing binary YY Gem ( P = 0.814 days ) , a member of the Castor sextuple system and one of the benchmarks for the comparison between observations and stellar evolution theory in the lower main-sequence .
In addition , we have re-analyzed existing light curves in several passbands using modern techniques that account for the conspicuous presence of spots .
This , combined with the spectroscopy , has yielded a very precise determination of the absolute dimensions of the components , which are virtually identical to each other .
We obtain for the mean mass , radius , and effective temperature the values M = 0.5992 \pm 0.0047 M _ { \sun } , R = 0.6191 \pm 0.0057 R _ { \sun } , and T _ { eff } = 3820 \pm 100 K. Both the mass and the radius determinations are good to better than 1 % , which in the case of the radius represents a fourfold improvement over previous results and significantly enhances the value of this quantity for testing the models .
We discuss the importance of systematic effects in these measurements , by comparison with another high-precision determination of the mass by Ségransan et al .
( 2000 ) .
A re-analysis of the Hipparcos transit data for Castor AB that accounts for the relative motion of the pair in its 467 yr-period orbit has yielded an improved parallax for the system of 66.90 \pm 0.63 mas .
With this , we have estimated the age ( \sim 370 Myr ) and metal abundance ( [ Fe/H ] \sim 0.0 ) of YY Gem from isochrone fits to Castor A and B under the assumption of a common origin .
This , along with the other physical properties , allows for an unusually stringent test of the models for low-mass stars .
We have compared the observations of YY Gem with a large number of recent theoretical calculations , and we show that all models underestimate the radius by up to 20 % , and most overestimate the effective temperature by 150 K or more .
Both of these trends are confirmed by observations of another similar system in the Hyades cluster ( V818 Tau ) .
Consequently , theoretical ages for relatively low-mass objects such as T Tauri stars derived by placing them on the H-R diagram may be considerably biased .
If the radius is used directly as a measure of evolution , ages could be underestimated by as much as a factor of 10 in this mass regime .
In view of these discrepancies , absolute ages from essentially all current models for the lower main sequence must be viewed with at least some measure of skepticism .
Finally , we derive a new and very accurate ephemeris based on all available times of eclipse , and we lay to rest previous claims of sudden changes in the orbital period of the binary , which we show to be spurious .