  We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio,
a diagnostic of the chemical history of the Solar Neighbourhood. Our sample of
K dwarfs has homogeneously determined effective temperatures, bolometric
luminosities and metallicities, allowing us to fit each star to the appropriate
stellar isochrone and determine its helium content indirectly. We use a newly
computed set of Padova isochrones which cover a wide range of helium and metal
content. Our theoretical isochrones have been checked against a congruous set
of main sequence binaries with accurately measured masses, to discuss and
validate their range of applicability. We find that the stellar masses deduced
from the isochrones are usually in excellent agreement with empirical
measurements. Good agreement is also found with empirical mass-luminosity
relations. Despite fitting the masses of the stars very well, we find that
anomalously low helium content (lower than primordial helium) is required to
fit the luminosities and temperatures of the metal poor K dwarfs, while more
conventional values of the helium content are derived for the stars around
solar metallicity. We have investigated the effect of diffusion in stellar
models and LTE assumption in deriving metallicities. Neither of these is able
to resolve the low helium problem alone and only marginally if the cumulated
effects are included, unless we assume a mixing-length which is strongly
decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be Delta Y / Delta Z = 2.1 +/-
0.9 around and above solar metallicity, consistent with previous studies,
whereas open problems still remain at the lowest metallicities. Finally, we
determine the helium content for a set of planetary host stars.
