He and Si surface inhomogeneities of four Bp variable stars

We present ground-based multi-colour Geneva photometry and high-resolution spectra of four variable B-type stars: HD105382, HD131120, HD138769 and HD55522. All sets of data reveal monoperiodic stars. A comparison of moment variations of two spectral lines, one silicon line and one helium line, allows us to exclude the pulsation model as being the cause of the observed variability of the four stars. We therefore delete the four stars from the list of candidate slowly pulsating B stars. We attribute the line-profile variations to non-homogeneous distributions of elements on the stellar surface and we derive abundance maps for both elements on the stellar surface by means of the Doppler Imaging technique. We confirm HD131120 to be a He-weak star and we classify HD105382, HD138769 as new He-weak stars. HD55522 has the solar helium abundance but the mean abundance value of He varies by 0.8 dex during the stellar rotation. For HD131120 and HD105382, helium is enhanced in regions of the stellar surface where silicon is depleted and depleted in regions where silicon is enhanced.