Light-curve inversions with truncated least-squares principal components: Tests and application to HD 291095=V1355 Orionis

We present a new inversion code that reconstructs the stellar surface spot configuration from the light curve of a rotating star. Our code employs a method that uses the truncated least-squares estimation of the inverse problem's objects principal components. We use spot filling factors as the unknown objects. Various test cases that represent a rapidly-rotating K subgiant are used for the forward problem. Tests are then performed to recover the artificial input map and include data errors and input-parameter errors. We demonstrate the robustness of the solution to false input parameters like photospheric temperature, spot temperature, gravity, inclination, unspotted brightness and different spot distributions and we also demonstrate the insensitivity of the solution to spot latitude. Tests with spots peppered over the entire stellar surface or with phase gaps do not produce fake active longitudes. The code is then applied to ten years of V and I-band light curve data of the spotted sub-giant HD 291095. A total of 22 light curves is presented. We find that for most of the time its spots were grouped around two active longitudes separated on average by 180 degrees. Switches of the dominant active region between these two longitudes likely occurred about every 3.15 circle dot 0.23 years while the amplitude modulation of the brightness occurred with a possible period of 3.0 circle dot 0.15 years. For the first time, we found evidence that the times of the activity flips coincide with times of minimum light as well as minimum photometric amplitude, i.e. maximum spottedness. From a comparison with simultaneous Doppler images we conclude that the activity flips likely take place near the rotational pole of the star. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.