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tokens: loud (GL) sample]. The RoboPol polarimeter allows for the simultaneous measurement of the Stokes q and u vectors without any moving parts. Each source on the sky is projected four times (four spots in a cross-like pattern) on the CCD camera each having its polarization orientation rotated by 67.5°. This allows for measurement of the polarization degree and EVPA of a source in a single exposure. For enhanced sensitivity and to block unwanted light, a mask is placed in the centre of the CCD, the location where the source of interest is typically positioned. A consequence of the RoboPol design is that sources outside the mask could have one (or more) of their spots overlap with a neighbouring source preventing us from confidently measuring either source’s polarization parameters (see fig. 6 in Panopoulou et al. 2015 ), especially in crowded fields. For every blazar observed with the RoboPol polarimeter, we use the automated reduction pipeline to extract the polarization degree for the sources in the 13 arcmin ×13 arcmin field of view of the RoboPol instrument excluding sources affected by such overlap (for details on the RoboPol pipeline and reduction process, see King et al. 2014 ; Panopoulou et al. 2015 ). This process yielded polarization measurements for 5837 sources. Additionally, we include 6379 sources from the Heiles ( 2000 ) catalogue of polarized stars residing above and below the galactic plane (| b | > 10° and | b | −10°) resulting in a total of 12 216 sources [hereafter γ-ray quiet (GQ) sample]. Fig. 1 shows the median polarization degree of the GL and GQ samples. Clearly, the GL sources show systematically higher polarization due to the synchrotron nature of their emission as opposed to the, most likely, dust-induced polarization of the emission in GQ sources. Figure 1. Optical polarization degree distribution for the γ-ray loud and γ-ray quiet samples. We use the Wide-field Infrared Survey Explorer ( WISE ) all-sky survey 4 and the Two Micron All Sky Survey (2MASS, Skrutskie et al. 2006 to obtain the 3.4, 4.6, and 12 μm, and H -band magnitudes for all the sources in the GL and GQ samples. The WISE colours have been successfully used in the past to find blazars among the unidentified Fermi sources (e.g. Massaro et al. 2011 ). Fig. 2 shows the WISE colour–colour diagram along with the so-called blazar strip found in Massaro et al. ( 2011 , 2012 ). While there is some minor overlap, the GL sample occupies a very distinct area in the infrared colour–colour diagram. We also estimate the H -band–4.6 μm colour. In Majewski, Zasowski Nidever ( 2011 ) stars were shown to have a narrow distribution. The GQ sample (which is mostly comprised of stars) shows a similar narrow distribution, whereas GL sources show a wider distribution shifted to higher values (Fig. 3 ). In addition, stars and nearby galactic sources could have measurable proper motion while we do not expect any significant proper motion in distant extragalactic sources (i.e.