Context :

Aims : RefPlanets is a guaranteed time observation ( GTO ) programme that uses the Zurich IMaging POLarimeter ( ZIMPOL ) of SPHERE/VLT for a blind search for exoplanets in wavelengths from 600-900 nm .
The goals of this study are the characterization of the unprecedented high polarimetic contrast and polarimetric precision capabilities of ZIMPOL for bright targets , the search for polarized reflected light around some of the closest bright stars to the Sun and potentially the direct detection of an evolved cold exoplanet for the first time .

Methods : For our observations of \alpha Cen A and B , Sirius A , Altair , \epsilon Eri and \tau Ceti we used the polarimetric differential imaging ( PDI ) mode of ZIMPOL which removes the speckle noise down to the photon noise limit for angular separations \gtrapprox 0.6″ .
We describe some of the instrumental effects that dominate the noise for smaller separations and explain how to remove these additional noise effects in post-processing .
We then combine PDI with angular differential imaging ( ADI ) as a final layer of post-processing to further improve the contrast limits of our data at these separations .

Results : For good observing conditions we achieve polarimetric contrast limits of 15.0–16.3 mag at the effective inner working angle of \sim 0.13″ , 16.3–18.3 mag at 0.5″and 18.8–20.4 mag at 1.5″ .
The contrast limits closer in ( \lessapprox 0.6″ ) depend significantly on the observing conditions , while in the photon noise dominated regime ( \gtrapprox 0.6″ ) , the limits mainly depend on the brightness of the star and the total integration time .
We compare our results with contrast limits from other surveys and review the exoplanet detection limits obtained with different detection methods .
For all our targets we achieve unprecedented contrast limits .
Despite the high polarimetric contrasts we are not able to find any additional companions or extended polarized light sources in the data that has been taken so far .

Conclusions :