PROPERTIES OF HE-RICH STARS .1. THEIR EVOLUTIONARY STATE AND HELIUM ABUNDANCE

A determination of the surface gravity and an abundance analysis of he lium in a sample of 17 He-rich and 5 normal, reference stars is presen ted. These results are derived from low resolution CCD spectra, but ea ch star was measured at least 6 times in order to obtain a significant average spectrum for the spectroscopic variables. The helium abundanc es derived from the models used are very close to 0.1 for normal, refe rence stars and are larger for the others, clearly indicating the He-r ich phe nomenon in them. NLTE effects, errors on the microturbulence v alue or on the surface gravity do not influence the estimated helium a bundances. Nevertheless; synthesized Geneva colours are affected by th e He-rich peculiarity, especially the [U-B] index which systematically changes by -0.025 mag per 0.1 of He abundance for the coolest stars i n the sample. We cannot confirm the correlation between the evolutiona ry state and the helium abundance reported previously (Zboril et al. 1 994), although we used a more reliable technique of log g determinatio n. All He-rich objects lie within the main sequence: their surface gra vities are all inside the range 4.1 < log g < 4.5, with no more than t hree objects having log g < 4.25. We find a significant spread of heli um abundances in this range of surface gravities, from the solar value similar to 0.1 up to about 0.4. Some of the programme stars (includin g reference stars) present emission in their Balmer lines and therefor e some kind of stellar activity. Strong helium overabundance often coe xists with emission and stellar activity.