Upper panel: V-band light curve of J1023 (magnitude vs. orbital phase). The solid line represents the fit of the light curve with a sinusoidal function with period fixed to 1 (corresponding to 4.75 h; Archibald et al. 2009). Bottom panel: V-band polarisation curve represented as a function of the orbital phase of the system. The >1σ detections are represented with red triangles, whereas we indicated the 3σ upper limits with red arrows, where necessary. The fit of the >1σ detections with a constant function is superimposed (dashed line; χ²/ d.o.f. = 0.28/3). Errors are represented at the 68% confidence level. Two periods are drawn for clarity.