  Context: It has been speculated for many years that some extrasolar planets
may emit strong cyclotron emission at low radio frequencies in the range 10-100
MHz. Despite several attempts no such emission has yet been seen.
  Aims: The hot Jupiter system tau Bootis is one of the nearest (d=15 pc)
exoplanets known to date. The gravitational influence of this massive hot
Jupiter (M=6 M_jup) has locked the star-planet system, making the star rotate
in P~3.3 days, similar to the orbital period of the planet. From the well
established correlation between stellar rotation and radio luminosity, it is
conceivable that the tau Bootis system emits strong radio emission at
significantly higher frequencies than currently probed, which we aimed to
investigate with this work.
  Methods: We observed tau Bootis with the Westerbork Synthesis Radio Telescope
(WSRT) at a frequency of 1.7 GHz. for 12 hours in spectral line mode, reaching
a noise level of 42 microJy/beam at the position of the target.
  Results: No 18cm radio emission is detected from tau Bootis, resulting in a 3
sigma upper limit of 0.13 mJy, corresponding to a 18cm radio luminosity of
<3.7e13 erg/s/Hz. We observe tau Bootis to be two orders of magnitude fainter
than expected from the stellar relation between radio luminosity and rotation
velocity.
  Conclusions: This implies that either the tau Bootis system is underluminous
in the radio compared to similar fast-rotating stars, or that we happened to
observe the target during a low state of radio emission.
