  Context: Rotation plays a key role in the life cycles of stars with masses
above 8 Msun. Hence, accurate knowledge of the rotation rates of such massive
stars is critical for understanding their properties and for constraining
models of their evolution. Aims: This paper investigates the reliability of
current methods used to derive projected rotation speeds v sin i from
line-broadening signatures in the photospheric spectra of massive stars,
focusing on stars that are not rapidly rotating. Methods: We use slowly
rotating magnetic O-stars with well-determined rotation periods to test the
Fourier transform (FT) and goodness-of-fit (GOF) methods typically used to
infer projected rotation rates of massive stars. Results: For our two magnetic
test stars with measured rotation periods longer than one year, i.e., with v
sin i < 1 km/s, we derive v sin i ~ 40-50 km/s from both the FT and GOF
methods. These severe overestimates are most likely caused by an insufficient
treatment of the competing broadening mechanisms referred to as microturbulence
and macroturbulence. Conclusions: These findings warn us not to rely
uncritically on results from current standard techniques to derive projected
rotation speeds of massive stars in the presence of significant additional line
broadening, at least when v sin i <~ 50 km/s. This may, for example, be crucial
for i) determining the statistical distribution of observed rotation rates of
massive stars, ii) interpreting the evolutionary status and spin-down histories
of rotationally braked B-supergiants, and iii) explaining the deficiency of
observed O-stars with spectroscopically inferred v sin i ~ 0 km/s. Further
investigations of potential shortcomings of the above techniques are presently
under way.
