Investigating the surface inhomogeneities of the contact binary SW Lacertae I. Doppler imaging

Aims. We aim to reconstruct the first detailed surface maps of the W UMa-type contact binary system, SW Lac. These maps should reveal the distributions of dark, magnetically active spot regions on the component stars and enable us to compare these with the results of similar studies of other active stars. Methods. We used the noise-reduction technique least squares deconvolution (LSD) to obtain high signal-to-noise ratio spectra of SW Lac, enabling individual starspot features to be observed in the highly rotationally broadened profiles of this rapid rotator. We performed the Doppler mapping of the system using the Doppler imaging code, DoTS. To test the reliability of our images, we assessed the performance of the code when applied to data with incomplete phase coverage. Results. We obtained surface maps of the system. The secondary (more massive) component is found to have a lower effective temperature and a slightly higher spot coverage than the primary. These may indicate that the secondary is more spotted than the primary component. Our results are consistent with theoretical assumptions as well as the photometric studies of W-type W UMa contact binary systems, indicating that the secondary component is typically more active than its primary.