We present high-quality optical (V) observations of a transit of planetary candidate 
OGLE-TR-171. The observations were performed with VIMOS at the European Southern Observatory 
(ESO) Very Large Telescope (VLT). Our photometry is about 10 times more precise and 100 
times more frequent than the OGLE photometry, and that is achieved accumulating more photons 
faster, using a large telescope.
                      
This allowed to measure the eclipse amplitude with a precision of about 0.001 mag. We also 
obtained an accurate transit duration and determined the time of mean transit. These 
parameters were used to obtain a precise estimate of the transit geometry (impact parameter 
and orbital inclination angle) and the planetary radius.
                     
Our observations can be complemented with previous ones done in other bands, like OGLE I 
band, to obtain furtheri nformation about the transiting companion.