Dissecting the donor star in the eclipsing polar HU Aquarii

Medium-resolution spectroscopic observations with full phase coverage of the eclipsing polar HU Aqr are presented. Na i absorption and Ca ii emission lines are used to trace the secondary star. While the Na i lines seem to have contributions from both hemispheres of the donor star, the Ca ii lines were found to originate from the irradiated part of its surface alone. An irradiation model was applied and the irradiation-weighted radial velocity amplitude was used to determine the mass ratio Q = MWD/M2 = 4.58 ± 0.20. When combined with high-speed photometric low-state eclipse light curves, the white dwarf mass is constrained to 0.75 < MWD/M < 0.84 with a best-fit value of MWD = (0.80 ± 0.04) M , and M2 = (0.18 ± 0.06) M at an orbital inclination of i = 87◦+0.8 −0.5.