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Context: Rotation plays a key role in the life cycles of stars with massesabove 8 Msun. Hence, accurate knowledge of the rotation rates of such massivestars is critical for understanding their properties and for constrainingmodels of their evolution. Aims: This paper investigates the reliability ofcurrent methods used to derive projected rotation speeds v sin i fromline-broadening signatures in the photospheric spectra of massive stars,focusing on stars that are not rapidly rotating. Methods: We use slowlyrotating magnetic O-stars with well-determined rotation periods to test theFourier transform (FT) and goodness-of-fit (GOF) methods typically used toinfer projected rotation rates of massive stars. Results: For our two magnetictest stars with measured rotation periods longer than one year, i.e., with vsin i < 1 km/s, we derive v sin i ~ 40-50 km/s from both the FT and GOFmethods. These severe overestimates are most likely caused by an insufficienttreatment of the competing broadening mechanisms referred to as microturbulenceand macroturbulence. Conclusions: These findings warn us not to relyuncritically on results from current standard techniques to derive projectedrotation speeds of massive stars in the presence of significant additional linebroadening, at least when v sin i <~ 50 km/s. This may, for example, be crucialfor i) determining the statistical distribution of observed rotation rates ofmassive stars, ii) interpreting the evolutionary status and spin-down historiesof rotationally braked B-supergiants, and iii) explaining the deficiency ofobserved O-stars with spectroscopically inferred v sin i ~ 0 km/s. Furtherinvestigations of potential shortcomings of the above techniques are presentlyunder way.

The rotation rates of massive stars: How slow are the slow ones?