  Context. A new four-telescope interferometric instrument called PIONIER has
recently been installed at VLTI. It provides improved imaging capabilities
together with high precision. Aims. We search for low-mass companions around a
few bright stars using different strategies, and determine the dynamic range
currently reachable with PIONIER. Methods. Our method is based on the closure
phase, which is the most robust interferometric quantity when searching for
faint companions. We computed the chi^2 goodness of fit for a series of binary
star models at different positions and with various flux ratios. The resulting
chi^2 cube was used to identify the best-fit binary model and evaluate its
significance, or to determine upper limits on the companion flux in case of non
detections. Results. No companion is found around Fomalhaut, tau Cet and
Regulus. The median upper limits at 3 sigma on the companion flux ratio are
respectively of 2.3e-3 (in 4 h), 3.5e-3 (in 3 h) and 5.4e-3 (in 1.5 h) on the
search region extending from 5 to 100 mas. Our observations confirm that the
previously detected near-infrared excess emissions around Fomalhaut and tau Cet
are not related to a low-mass companion, and instead come from an extended
source such as an exozodiacal disk. In the case of del Aqr, in 30 min of
observation, we obtain the first direct detection of a previously known
companion, at an angular distance of about 40 mas and with a flux ratio of
2.05e-2 \pm 0.16e-2. Due to the limited u,v plane coverage, its position can,
however, not be unambiguously determined. Conclusions. After only a few months
of operation, PIONIER has already achieved one of the best dynamic ranges
world-wide for multi-aperture interferometers. A dynamic range up to about
1:500 is demonstrated, but significant improvements are still required to reach
the ultimate goal of directly detecting hot giant extrasolar planets.
