We present the first linear polarimetric survey of white dwarfs ( WDs ) .
Our sample consists of WDs of DA and DC spectral types in the SDSS r magnitude range from 13 to 17 .
We performed polarimetric observations with the RoboPol polarimeter attached to the 1.3-m telescope at the Skinakas Observatory .
We have 74 WDs in our sample , of which almost all are low polarized WDs with polarization degree ( PD ) smaller than 1 % , while only 2 have PD higher than 1 % .
There is an evidence that on average the isolated WDs of DC type have higher PD ( with median PD of 0.78 % ) than the isolated DA type WDs ( with median PD of 0.36 % ) .
On the other hand , the median PD of isolated DA type WDs is almost the same , i.e .
0.36 % as the median PD of DA type white dwarfs in binary systems with red dwarfs ( dM type ) , i.e .
0.33 % .
This shows , as expected , that there is no contribution to the PD from the companion if the WD companion is the red dwarf , which is the most common situation for WDs binary systems .
We do not find differences in the polarization degree between magnetic and non-magnetic WDs .
Because 97 % of WDs in our sample have PD lower than 1 % , they can be used as faint zero–polarized standard star in the magnitude range from 13 up to 17 of SDSS r filter .
They cover the Northern sky between 13 hour to 23 hour in right ascension and from -11 ^ { \circ } to 78 ^ { \circ } in declination .
Additionally , we found that for low extinction values ( < 0.04 ) the best model that describes the dependence of PD on E ( B–V ) is given by the equation : { PD _ { max,ISM } } [ \% ] = 0.65 ~ { } E ( B - V ) ^ { 0.12 } .