The discovery of the very hot exoplanet HD 189733b was announced by the team of F. Bouchy of the Observatoire de Haute Provence (France) on October 5th, 2005. His team, founded by Michel Mayor of Geneva Observatory (Switzerland), had started an exoplanet search programme biased towards high-metallicity stars, named ELODIE, in March 2004, using high-precision radial velocity measurements. HD 189733b is the 9th transiting exoplanet found to date. It is located in Vulpecula, just 0.3 degress from the Dumbell Nebula (M27).

HD 189733b was reported to be a transiting hot Jupiter, with a period of 2.219d, and with a photometric depth of about 3%. Considering that HD 189733, a type K-star, has the same visual magnitude as the well known exoplanet host star HD 209458, of which I succeeded to observe an exoplanet transit in September 2003, I decided to start photometric observations of HD 189733 at the first possible transit opportunity.  

  
2005, October 16/17

The night of October 16/17, 2005 presented such a possibility. Observing conditions were far from ideal : full moon, low altitude of the star at the moment of the ingress (approx 40d), cirrus near the start of the transit and clouds towards the end of the session. I started V-filtered CCD observations at 20h48m UT (Oct 16.867 UT), about 1 hour in advance of the predicted time of ingress. Mid-transit of HD 189733b was expected at 22h55m UT. The session was stopped after 2h and 140 useful CCD images. At that moment, HD 189733 was less than 30 degrees above my local horizon, hence too low for millimag photometry.

As comparison star, I used SAO 88041, a G0-type star of mag 8.94, about 8'40" (P.A. 267°) from HD 189733. Exposure times varied from 15 to 30 sec, and initially slight defocusing was applied when necessary to avoid saturation.

 
Analysis of the observations

Data analysis was done using the aperture photometry function of MIRA AP, on non-stacked images. I used the S/N values produced by MIRA to eliminate poor quality images. I then used Peranso to create a 'binned' lightcurve, grouping observations in 9-min bins and indicating the standard deviation per bin. The final result is presented below.

Note : amateur astronomer Ron Bissinger (California) also succeeded in capturing a transit of HD 189733b at his private observatory, on Oct 10th, 2005. His observations are depicted below.