Upper panel: R-band light curve of J1023 (magnitude vs. orbital phase). With a solid line, a fit of the light curve with a sinusoidal function with period fixed to 1 (corresponding to 4.75 h; Archibald et al. 2009) is reported. The χ²/ d.o.f. of the fit is 11.35/17. Bottom panel: R-band polarisation curve represented with respect to the orbital phase of the system. The fits with a constant function (dashed line; χ²/ d.o.f. = 3.66/4) and with a sinusoidal function with period fixed to 1 are superimposed (solid line; χ²/ d.o.f. = 0.11/2). Errors are represented at the 68% confidence level. Two periods are drawn for clarity.