  The aim of the KArlsruhe TRItium Neutrino experiment KATRIN is the
determination of the absolute neutrino mass scale down to 0.2 eV, with
essentially smaller model dependence than from cosmology and neutrinoless
double beta decay. For this purpose, the integral electron energy spectrum is
measured close to the endpoint of molecular tritium beta decay. The endpoint,
together with the neutrino mass, should be fitted from the KATRIN data as a
free parameter. The right-handed couplings change the electron energy spectrum
close to the endpoint, therefore they have some effect also to the precise
neutrino mass determination. The statistical calculations show that, using the
endpoint as a free parameter, the unaccounted right-handed couplings
constrained by many beta decay experiments can change the fitted neutrino mass
value, relative to the true neutrino mass, by not larger than about 5-10 %.
Using, incorrectly, the endpoint as a fixed input parameter, the above change
of the neutrino mass can be much larger, order of 100 %, and for some cases it
can happen that for large true neutrino mass value the fitted neutrino mass
squared is negative. Publications using fixed endpoint and presenting large
right-handed coupling effects to the neutrino mass determination are not
relevant for the KATRIN experiment.
