Light curves have long been used to study stellar activity and have more recently become a major tool in the field of exoplanet research. We discuss the various ways in which stellar activity can influence transit light curves, and study the effects using the outstanding photometric data of the CoRoT-2 exoplanet system. We report a relation between the "global" light curve and the transit profiles, which turn out to be shallower during high spot coverage on the stellar surface. Furthermore, our analysis reveals a color dependence of the transit light curve compatible with a wavelength-dependent limb darkening law as observed on the Sun. Taking into account activity-related effects, we redetermine the orbit inclination and planetary radius and find the planet to be ≈3% larger than reported previously. Our findings also show that exoplanet research cannot generally ignore the effects of stellar activity.

The center-to-limb variation (CLV) describes the brightness of the stellar disk as a function of the limb angle. Across strong absorption lines, the CLV can vary quite significantly. We obtained a densely sampled time series of high-resolution transit spectra of the active planet host star HD 189733 with UVES. Using the passing planetary disk of the hot Jupiter HD 189733 b as a probe, we study the CLV in the wings of the Ca  H and K and Na  D 1 and D 2 Fraunhofer lines, which are not strongly affected by activity-induced variability.In agreement with model predictions, our analysis shows that the wings of the studied Fraunhofer lines are limb brightened with respect to the (quasi-)continuum. The strength of the CLV-induced effect can be on the same order as signals found for hot Jupiter atmospheres. Therefore, a careful treatment of the wavelength dependence of the stellar CLV in strong absorption lines is highly relevant in the interpretation of planetary transit spectroscopy.

Chromospheric Ca II activity cycles are frequently found in late-type stars, but there have been no systematic programs to search for their coronal X-ray counterparts. The typical time scale of Ca II activity cycles goes from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. XMM-Newton has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star ǫ Eridani, observed previously twice in 2003 and in early 2015 by XMM-Newton. With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for ǫ Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term XMM-Newton lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity results to be 2 × 10 28 erg/s, with an amplitude that is only a factor 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of ǫ Eridani in terms of solar magnetic structures: active regions, cores of active regions and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can describe the observed X-ray emission measure of ǫ Eridani only if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of ǫ Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of ǫ Eridani. Our analysis revealed also that the X-ray cycle of ǫ Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of ǫ Eridani is thus explained by the high percentage of magnetic structures on its surface.

TIGRE is a new robotic spectroscopy telescope located in central Mexico at the La Luz Observatory of the University of Guanajuato. The 1.2 m telescope is fiber-coupled to anéchelle spectrograph with a spectral resolving power exceeding 20 000 over most of the covered spectral range between 3800Å and 8800Å, with a small gap of 130Å around 5800 A. TIGRE operates robotically, i.e. it (normally) carries out all observations without any human intervention, including, in particular, the target selection in any given observing night. In this paper we describe the properties of the TIGRE instrumentation and its technical realization, as well as our first operational experience with the performance and efficiency of the overall system. Finally, we present some examples of recent TIGRE observations.

Context. Planetary transit light curves are influenced by a variety of fundamental parameters, such as the orbital geometry and the surface brightness distribution of the host star. Stellar limb darkening (LD) is therefore among the key parameters of transit modeling. In many applications, LD is presumed to be known and modeled based on synthetic stellar atmospheres. Aims. We measure LD in a sample of 38 Kepler planetary candidate host stars covering effective temperatures between 3000 K and 8900 K with a range of surface gravities from 3.8 to 4.7. In our study we compare our measurements to widely used theoretically predicted quadratic limb-darkening coefficients (LDCs) to check their validity. Methods. We carried out a consistent analysis of a unique stellar sample provided by the Kepler satellite. We performed a Markov chain Monte Carlo (MCMC) modeling of low-noise, short-cadence Kepler transit light curves, which yields reliable error estimates for the LD measurements in spite of the highly correlated parameters encountered in transit modeling. Results. Our study demonstrates that it is impossible to measure accurate LDCs by transit modeling in systems with high impact parameters (b 0.8). For the majority of the remaining sample objects, our measurements agree with the theoretical predictions, considering measurement errors and mutual discrepancies between the theoretical predictions. Nonetheless, theory systematically overpredicts our measurements of the quadratic LDC u 2 by about 0.07. Systematic errors of this order for LDCs would lead to an uncertainty on the order of 1% for the derived planetary parameters. Conclusions. We find that it is adequate to set the commonly used theoretical LDCs as fixed parameters in transit modeling. Furthermore, it is even indispensable to use theoretical LDCs in the case of transiting systems with a high impact parameter, since the host star's LD cannot be determined from their transit light curves.