Millimagnitude Optical Photometry for the Transiting Planetary Candidate OGLE‐TR‐109

We present precise V-band photometry for the low-amplitude transit candidatestar OGLE-TR-109. This is an extreme case among the transiting candidates foundby the OGLE group because of the early spectral type of the star (F0V), of thelow transit amplitude (A_I=0.008 mag), and of the very short period (P=0.58909days) of the orbiting companion. Using difference image photometry, we are ableto achieve millimagnitude errors in the individual data points. One transit ofthis star is well defined in our light curve. This confirms the OGLE detectionand rules out the possibility of a false positive. The measurement of thistransit allows to refine the transit amplitude (A_V=0.006 +/- 0.001 mag), andthe ephemerides for this interesting system, as well as the radius of thepossible orbiting companion (R_P=0.90 +/- 0.09 ~R_J), and the inclination ofthe orbit (i=77 +/- 5 deg). Two other transits observed at lower S/N confirmthe period of this system measured by OGLE. There is no evidence for a blend ofthe F-type main sequence star with a redder eclipsing binary, or for secondarytransits in the present observations. The absence of ellipsoidal modulation inthe light curve of the primary rules out a low mass star companion or browndwarf with M>14 +/- 8 M_J. The remaining possibilities for OGLE-TR-109 are ablend between the F-type star and a binary with a bluer primary star, or a newtransiting extrasolar planet.Comment: 24 pages (including figures) submitteed to ApJ, Accepted. 1 replacement (updated references