tokens: 90072 two more transit windows of HD 10180 b, this time at 4.5 m. No transit structure was clearly detected in the resulting light curves (Fig. 4 , bottom), and a short MCMC analysis of the data led us to conclude that these new data do not increase the significance of the 2010 tentative detection. We thus conclude that the low-amplitude structure in our Cycle 6 Spitzer light curve is probably just correlated noise of instrumental origin that is not perfectly represented by our baseline model and that can easily be modeled by a shallow enough transit profile. For Spitzer , the average amplitude of these correlated noise structures is of a few dozen of ppm (see Sect. 6), making the firm detection of a unique transit shallower than 100 ppm impossible – similarly to the one expected for HD 10180 b (see discussion in Sect. 6 and our estimated detection thresholds in Table 1 ). This so-called red noise limit can be surpassed, but only by gathering more observations of the transit window, as we did here (see also the case of HD 40307 b in Gillon et al. 2010 ). Fig. 3 Same as Fig. 1 for the light curve of 2010 Jan. 16 for HD 10180 c. Fig. 4 Top : same as Fig. 3 , except that a possible transit of HD 10180 b was included in the global model. Highest posterior probability transit model for HD 10180 b is shown in purple. The posterior PDF for the transit timing of HD 10180 b derived from an RV analysis assuming 7 planets is shown above the figure. Bottom : 2013 Warm Spitzer photometry for HD 10180 binned per 30 min, corrected for the instrumental effects, normalized, and folded on the transit ephemeris for HD 10180 b based on our tentative detection of a transit and on the results of an RV analysis assuming 7 planets (d = time from most likely mid-transit time). 5.4. HD 13808 In 2012, we analyzed the HARPS dataset for the early K-dwarf HD 13808, confirming the existence of the two planets around it with periods of 14.2 d and 53.7 d announced by Mayor et al. ( 2011 , hereafter M11). Furthermore, our analysis revealed the existence of (1) a low-frequency signal that was well-modeled with a quadratic trend, whose origins is the magnetic cycle of the star (Queloz et al., in prep.). and (2) a low-amplitude Doppler signal with a period of 1.091 d which corresponds to a planet of with an interestingly high transit probability of 20%. The false-alarm probability (FAP) derived by Systemic for this short-period planet candidate was close to 1%. Based on this small FAP and the scientific importance of this putative planet if transiting, we decided to monitor two of its transit windows with Spitzer . Unfortunately, the resulting light curves did not show any convincing transit-like structure (Fig. 5 ). Our Spitzer data were not acquired during a transit window of the planet at 14.2 d, thus keeping its transiting nature unconstrained. In 2014, we analyzed the updated HARPS dataset. This analysis confirmed the existence of the low-frequency signal of magnetic cycle
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