Aims: We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24. Methods: We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar

Due to stellar rotation, the observed radial velocity of a star varies during the transit of a planet across its surface, a phenomenon known as the Rossiter–McLaughlin (RM) effect. The amplitude of the RM effect is related to the radius of the planet which, because of differential absorption in the planetary atmosphere, depends on wavelength

Context. Because of its proximity, Mrk 421 is one of the best sources on which to study the nature of BL Lac objects. Its proximity allows us to characterize its broadband spectral energy distribution (SED). Aims: The goal is to better understand the mechanisms responsible for the broadband emission and the temporal evolution of Mrk 421. These

We present updates to PRISM, a photometric transit-starspot model, and GEMC, a hybrid optimization code combining MCMC and a genetic algorithm. We then present high-precision photometry of four transits in the WASP-6 planetary system, two of which contain a starspot anomaly. All four transits were modelled using PRISM and GEMC, and the physical

Context. M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015 - the first event of this kind observed in M 31 since 1988. Very few similar events have been confirmed as of 2015. Most of them are considered to be products of stellar mergers. Aims: Results of an extensive optical monitoring of the transient in the period January

Aims: We observed the τ Boo system with the HARPS-N spectrograph to test a new observational strategy aimed at jointly studying asteroseismology, the planetary orbit, and star-planet magnetic interaction. Methods: We collected high-cadence observations on 11 nearly consecutive nights and for each night averaged the raw FITS files using a dedicated