Context :

Aims : Thanks to observations performed at the Haute-Provence Observatory , Bouchy et al .
recently announced the detection of a 2.2-day orbital period extra-solar planet that transits the disk of its parent star , HD 189733 .
We searched in the Hipparcos photometry Catalogue possible detections of those transits .

Methods : Statistic studies were performed on the Hipparcos data in order to detect transits of HD 189733 b and to quantify the significance of their detection .

Results : With high level of confidence , we find that Hipparcos likely observed one transit of HD 189733 b in October 1991 , and possibly two others in February 1991 and February 1993 .
Using the range of possible periods for HD 189733 b , we find that the probability that none of those events are due to planetary transits but are instead all due to artifacts is lower than 0.15 % .
Thanks to the 15-year temporal baseline available , we can measure the orbital period of the planet HD 189733 b with a particularly high accuracy .
We obtain a period of 2.218574 ^ { +0.000006 } _ { -0.000010 } days , corresponding to an accuracy of \sim 1 second .
Such accurate measurements might provide clues for companions presence .

Conclusions :