  Water, methane and carbon-monoxide are expected to be among the most abundant
molecules besides molecular hydrogen in the hot atmosphere of close-in EGPs.
Transit observations in the mid-IR allow the atmospheric content of transiting
planets to be determined. We present new primary transit observations of the
hot-jupiter HD189733b, obtained simultaneously at 4.5 and 8 micron with IRAC
instrument onboard Spitzer. Together with a new refined analysis of previous
observations at 3.6 and 5.8 micron using the same instrument, we are able to
derive the system parameters, including planet-to-star radius ratio, impact
parameter, scale of the system, and central time of the transit from fits of
the transit light curves at these four wavelengths. We measure the four
planet-to-star radius ratios, to be (R_p/R_*)= 0.1545 +/- 0.0003, 0.1557 +/-
0.0003, 0.1547 +/- 0.0005, 0.1544 +/- 0.0004 at 3.6, 4.5, 5.8, and 8 micron
respectively. The high accuracy of the measurement allows the search for
atmospheric molecular absorbers. Contrary to a previous analysis of the same
dataset, our study is robust against systematics and reveals that water vapor
absorption at 5.8 micron is not detected in this photometric dataset.
Furthermore, in the band centered around 4.5 micron we find a hint of excess
absorption with an apparent planetary radius Delta(R_p/R_*) = 0.00128 +/-
0.00056 larger (2.3 sigma) than the one measured simultaneously at 8 micron.
This value is 4 sigma above what would be expected for an atmosphere where
water vapor is the only absorbing species in the near infrared. This shows that
an additional species absorbing around 4.5 micron could be present in the
atmosphere. CO being a strong absorber at this wavelength is a possible
candidate and this may suggest a large CO/H2O ratio between 5 and 60.
