  The detection of transits is an efficient technique to uncover faint
companions around stars. The full characterisation of the companions (M-type
stars, brown dwarfs or exoplanets) requires high-resolution spectroscopy to
measure properly masses and radii. With the advent of massive variability
surveys over wide fields, the large number of possible candidates makes such a
full characterisation impractical for all of them. We explore here a fast
technique to pre-select the most promising candidates using either near-IR
photometry or low resolution spectroscopy. We develop a new method based on the
well-calibrated surface brightness relation along with the correlation between
mass and luminosity for main sequence stars, so that not only giant stars can
be excluded but also accurate effective temperatures and radii can be measured.
The main source of uncertainty arises from the unknown dispersion of extinction
at a given distance. We apply this technique to our observations of a sample of
34 stars extracted from the 62 low-depth transits identified by OGLE during
their survey of some 10e5 stars stars in the Carina fields of the Galactic
disc. We infer that at least 78% of the companions of the stars which are well
characterised in this sample are not exoplanets. Stars OGLE-TR-105, OGLE-TR-109
and OGLE-TR-111 are the likeliest to host exoplanets and deserve
high-resolution follow-up studies. OGLE-TR-111 was very recently confirmed as
an exoplanet with M_planet = 0.53 +- 0.1 M_Jup (Pont et al. 2004), confirming
the efficiency of our method in pre-selecting reliable planetary transit
candidates.
