We present Roche tomograms of the secondary star in the dwarf nova system RU Pegasi derived from blue- and red-arm ISIS data taken on the 4.2-m William Herschel Telescope. We have applied the entropy landscape technique to determine the system parameters and obtained component masses of M1= 1.06 M⊙, M2= 0.96 M⊙, an orbital inclination angle of i= 43° and an optimal systemic velocity of γ= 7 km s−1. These are in good agreement with previously published values.
Our Roche tomograms of the secondary star show prominent irradiation of the inner Lagrangian point due to illumination by the disc and/or bright spot, which may have been enhanced as RU Peg was in outburst at the time of our observations. We find that this irradiation pattern is axisymmetric and confined to regions of the star which have a direct view of the accretion regions. This is in contrast to previous attempts to map RU Peg which suggested that the irradiation pattern was non-symmetric and extended beyond the terminator.
We also detect additional inhomogeneities in the surface distribution of stellar atomic absorption that we ascribe to the presence of a large starspot. This spot is centred at a latitude of ∼82° and covers approximately 4 per cent of the total surface area of the secondary. In keeping with the high-latitude spots mapped on the cataclysmic variables (CVs) AE Aqr and BV Cen, the spot on RU Peg also appears slightly shifted towards the trailing hemisphere of the star. Finally, we speculate that early mapping attempts which indicated non-symmetric irradiation patterns which extended beyond the terminator of CV donors could possibly be explained by a superposition of symmetric heating and a large spot.