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 2.3 \times 10 ^ { -3 } ( in 4 hours ) , 3.5 \times 10 ^ { -3 } ( in 3 hours ) and 5.4 \times 10 ^ { -3 } ( in 1.5 hour ) 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 30Â 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 2.05 \times 10 ^ { -2 } \pm 0.16 \times 10 ^ { -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 .