  The Rossiter-McLaughlin effect observed for transiting exoplanets often
requires prior knowledge of the stellar projected equatorial rotational
velocity. This is usually provided by measuring the broadening of spectral
lines, however this method has uncertainties as lines are also broadened by
velocity fields in the stellar photosphere known as macroturbulence. We have
estimated accurate rotational velocity values from asteroseismic analyses of
main sequence stars observed by Kepler. The rotational frequency splittings of
the detected solar-like oscillations of these stars are determined largely by
the near-surface rotation. These estimates have been used to infer the
macroturbulence values for 28 Kepler stars. Out of this sample, 26 stars were
used along with the Sun to obtain a new calibration between macroturbulence,
effective temperature and surface gravity. The new calibration is valid for the
temperature range 5200 to 6400 K and the gravity range 4.0 to 4.6 dex. A
comparison is also provided with previous macroturbulence calibrations. As a
result of this work, macroturbulence, and thus rotational velocity, can now be
determined with confidence for stars that do not have asteroseismic data
available. We present new spectroscopic rotational velocity values for the WASP
planet host stars, using high resolution HARPS spectra.
