  We compare the accurate empirical mass-luminosity (M-L) relation based on
five Hyades binary systems to predictions of stellar models calculated with
various input parameters (helium, metallicity, age) or physics (mixing-length
ratio, model atmosphere, equation of state, microscopic diffusion). Models
based on a helium content Ysim0.28 inferred from the dydz enrichment law are
more than 3sigma beyond the observations, suggesting that the initial helium
abundance is lower than expected from its supersolar metallicity. With the
photometric metallicity (FeH=0.144pm0.013 dex, Grenon (2000) we derive
Y=0.255\pm0.009. Because of the (Y,FeH) degeneracy in the M-L plane, the
uncertainty grows to Delta Y=0.013 if the metallicity from spectroscopy is
adopted (FeH=0.14pm0.05 dex, Cayrel de Strobel et al 1997). We use these
results to discuss the Hertzsprung-Russell (HR) diagram of the Hyades, in the
(Mv,B-V) plane, based on the very precise Hipparcos dynamical parallaxes.
Present models fit the tight observed sequence very well except at low
temperatures. In the low mass region of the HR diagram sensitive to the
mixing-length parameter (aMLT), the slope of the main sequence (MS) suggests
that aMLT could decrease from a solar (or even supersolar) value at higher mass
to subsolar values at low mass, which is also supported by the modeling of the
vB22 M-L relation. We find that the discrepancy at low temperatures (B-V\gtrsim
1.2) remains, even if an improved equation of state or better model atmospheres
are used. Finally, we discuss the positions of the stars at turn-off in the
light of their observed rotation rates and we deduce that the maximum age of
the Hyades predicted by the present models is sim650 Myr.
