  We present precise V-band photometry for the low-amplitude transit candidate
star OGLE-TR-109. This is an extreme case among the transiting candidates found
by the OGLE group because of the early spectral type of the star (F0V), of the
low transit amplitude (A_I=0.008 mag), and of the very short period (P=0.58909
days) of the orbiting companion. Using difference image photometry, we are able
to achieve millimagnitude errors in the individual data points. One transit of
this star is well defined in our light curve. This confirms the OGLE detection
and rules out the possibility of a false positive. The measurement of this
transit allows to refine the transit amplitude (A_V=0.006 +/- 0.001 mag), and
the ephemerides for this interesting system, as well as the radius of the
possible orbiting companion (R_P=0.90 +/- 0.09 ~R_J), and the inclination of
the orbit (i=77 +/- 5 deg). Two other transits observed at lower S/N confirm
the period of this system measured by OGLE. There is no evidence for a blend of
the F-type main sequence star with a redder eclipsing binary, or for secondary
transits in the present observations. The absence of ellipsoidal modulation in
the light curve of the primary rules out a low mass star companion or brown
dwarf with M>14 +/- 8 M_J. The remaining possibilities for OGLE-TR-109 are a
blend between the F-type star and a binary with a bluer primary star, or a new
transiting extrasolar planet.
