A photoelectric narrow-band index, I(4026), of the He I λ 4026 line has been observed with an echelle spectrometer for B-type stars in young clusters and associations.
Model-atmosphere computations of the Strömgren [c
1] index, the Hβ index, and I(4026) as functions of T
eff, g and the helium-to-hydrogen ratio have been used to derive these atmospheric parameters from the observed indices for each star.
For the OriOBIb, LacOBI and Sco-Cen associations 11 out of 48 stars in the temperature range from 14 000 to 20 000 K are found to be helium poor in the sense that the derived helium-to-hydrogen ratios lie in the range from 0.05 down to very low values. In the temperature range from 20 000 to 30 000 K no helium poor stars are found, but two stars in OriOBIb are helium rich. Both of these stars show rather strong variations of the I(4026) index.
All stars in h + χ Per and CepOBIII are apparently helium deficient compared to nearby field stars.