We use nearby K dwarf stars to measure the helium–to–metal enrichment ratio \Delta Y / \Delta Z , 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 \pm 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 .