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Photometric and polarimetric clues to the circumstellar environment of RY Lupi

Aims. We investigate the stellar and circumstellar properties of the bright southern T Tauri star RY Lup, a G-type star showing type III variability. Methods. We report simultaneous BV polarimetric and UBV photometric observations obtained during 12 consecutive nights on the 1.0 m and 50 cm telescopes of the European Southern Observatory at La Silla. We compare these data to models. Results. The polarization is high (≈3.0%) when the star is faint and red (V ≈ 12.0, B − V ≈ 1.3), and it is low (≈0.5%) when it is bright and bluer (V ≈ 11.0, B − V ≈ 1.1). The photometric and polarimetric variations share a common period of 3.75 d. Irregular light variations, larger at shorter wavelengths, are also superposed on the cyclic variations and may be due to processes different than the one producing the periodic variations. The linear polarization is produced by dust scattering in an asymmetric (flat) circumstellar envelope. The photometric and polarimetric variations can be explained with an almost edge-on circumstellar disk that is warped close to the star, where it interacts with the star’s magnetosphere. The inhomogeneous disk matter contained in the warp corotates with the star and partially occults it during part of the rotation period, which explains the dips in luminosity and the accompanying increase in polarization. All the information available on RY Lup is consistent with a system comprising a G8 star surrounded by an edge-on disk, and we find that the mass of RY Lup is Mstar/M� = 1.71 ± 0.43, while its age is (1.2 ± 0.4) × 10 7 yr.