The accelerating rotation of the magnetic He-weak star HD 142990

HD 142990 (V 913 Sco; B5 V) is a He-weak star with a strong surface magnetic field and a short rotation period (P_rot ∼ 1 d). Whilst it is clearly a rapid rotator, recent determinations of P_rot are in formal disagreement. In this paper, we collect magnetic and photometric data with a combined 40-yr baseline in order to re-evaluate P_rot and examine its stability. Both period analysis of individual data sets and O - C analysis of the photometric data demonstrate that P_rot has decreased over the past 30 yr, violating expectations from magnetospheric braking models, but consistent with behaviour reported for 2 other hot, rapidly rotating magnetic stars, CU Vir and HD 37776. The available magnetic and photometric time series for HD 142990 can be coherently phased assuming a spin-up rate \dot{P} of approximately -0.6 s yr^-1, although there is some indication that \dot{P} may have slowed in recent years, possibly indicating an irregular or cyclic rotational evolution.