1996A&A...310..164D

Eta Orionis (HD35411) is a multiple system which consists of at least four early B-type stars. In the present paper we focus on the double-lined spectroscopic eclipsing pair Aab, and in particular on component Ab, which is a remarkable intrinsically variable star. The presence of this variable in an eclipsing system enables us to determine accurately its mass, radius, inclination angle and rotation period. From the profile variations of the Si III 4552-4574 triplet of ηOriAb a period of 0.13days is deduced, which we interpret in terms of a non-radial pulsation mode. The mode is identified as an (l,m)=(4,-3) mode; the moment method for mode identification yields an inclination angle that is fully consistent with the orbital inclination. The two components of the Aab system have the same effective temperature, nearly equal masses of 11.0 and 10.6M_☉ respectively, and a radius ratio of about 1.20. Both occur inside the β Cephei instability strip. Component Aa is a nonvariable star, however, while component Ab appears to be a g-mode pulsator. A striking difference between both stars is that the primary is rotating subsynchronously, while the rotation velocity of the secondary is four times higher than corotation. We suggest that the higher rotation velocity of component Ab is due to transfer of angular momentum by the pulsation in a differentially rotating star.