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HD 142990 (V 913 Sco; B5 V) is a He-weak star with a strong surface magneticfield and a short rotation period ($P_{\rm rot} \sim 1$ d). While it is clearlya rapid rotator, recent determinations of $P_{\rm rot}$ are in formaldisagreement. In this paper we collect magnetic and photometric data with acombined 40-year baseline in order to re-evaluate $P_{\rm rot}$ and examine itsstability. Both period analysis of individual datasets and $O-C$ analysis ofthe photometric data demonstrate that $P_{\rm rot}$ has decreased over the past30 years, violating expectations from magnetospheric braking models, butconsistent with behaviour reported for 2 other hot, rapidly rotating magneticstars, CU Vir and HD 37776. The available magnetic and photometric time seriesfor HD 142990 can be coherently phased assuming a spin-up rate $\dot{P}$ ofapproximately $-0.6$ s/yr, although there is some indication that $\dot{P}$ mayhave slowed in recent years, possibly indicating an irregular or cyclicrotational evolution.

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