2015A&A...578L...3H

Massive B-type stars with strong magnetic fields and fast rotation are very rare and pose a mystery for theories of star formation and magnetic field evolution. Only two such stars, called σ Ori E analogues, were known until recently. A team involved in APOGEE, one of the Sloan Digital Sky Survey III programs, announced the discovery of two additional rigidly rotating magnetosphere stars, HD23478 and HD345439. The magnetic fields in these newly discovered σ Ori E analogues have not been investigated so far. In the framework of our ESO Large Programme and one normal ESO programme, we carried out low-resolution FORS2 spectropolarimetric observations of HD23478 and HD345439. In the measurements of hydrogen lines, we discover a rather strong longitudinal magnetic field of up to 1.5kG in HD23478 and up to 1.3kG using the entire spectrum. The analysis of HD345439 using four subsequent spectropolarimetric subexposures does not reveal a magnetic field at a significance level of 3σ. On the other hand, individual subexposures indicate that HD345439 may host a strong magnetic field that rapidly varies over 88min. The fast rotation of HD345439 is also indicated by the behaviour of several metallic and HeI lines in the low-resolution FORS2 spectra that show profile variations already on this short time-scale.