  Observations of low-mass companions for which the dynamical masses are well
constrained help to improve the calibration of evolutionary models. Such
observations thereby provide more confidence in the estimation of the mass of a
companion using the photometric methods expected for the next generation of
planet finder instruments. The commissioning of a new coronagraph at the Very
Large Telescope (VLT) was the occasion to test the performance of this
technique on the well-known object AB Dor A and its 0.09$M_\odot$ companion AB
Dor C. The purpose of this paper is to refine the photometric analysis on this
object and to provide an accurate photometric error budget. In addition to
coronagraphy, we calibrated the residual stellar halo with a reference star. We
used standard techniques for photometric extraction. The companion AB Dor C is
easily detected at 0.185" from the primary star, and its magnitudes in H and Ks
are in agreement with an M5.5 object, as already known from spectroscopic
observations. However, these new measurements make the earlier J-band
photometry less reliable. Finally, the comparison with evolutionary models
supports an age of (75\pm 25) Myr, contrary to previous analyses. These
observations demonstrate that coronagraphic observations can be more efficient
than direct imaging, not only to improve contrast, but also to provide a better
photometric estimation as long as a good calibration of the stellar halo is
achieved.

Astrophysics