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Using a simple model based on the characteristics of sunspots and faculae in solar active regions, the effects of surface brightness inhomogeneities on the position of the photocentre of the disk of a solar-like, magnetically active star, are studied. A general law is introduced, giving the maximum amplitude of the photocentre excursion produced by a distribution of active regions with a given surface filling factor. The consequences for the detection of extrasolar planets by means of the astrometric method are investigated with some applications to forthcoming space missions, such as GAIA and SIM, as well as to ground-based interferometric measurements. Spurious detections of extrasolar planets can indeed be caused by activity-induced photocentre oscillations, requiring a simultaneous monitoring of the optical flux and a determination of the rotation period and of the level of activity of the target stars for an appropriate discrimination., Comment: 17 pages, 6 figures, accepted by New Astronomy

The space missions MOST, COROT and Kepler are going to provide us with high-precision optical photometry of solar-like stars with time series extending from tens of days to several years. They can be modelled to obtain information on stellar magnetic activity by fitting the rotational modulation of the stellar flux produced by the brightness inhomogeneities associated with photospheric active regions. The variation of the total solar irradiance provides a good proxy for those photometric time series and can be used to test the performance of different spot modelling approaches. We test discrete spot models as well as maximum entropy and Tikhonov regularized spot models by comparing the reconstructed total sunspot area variation and longitudinal distributions of sunspot groups with those actually observed in the Sun along activity cycle 23. Appropriate statistical methods are introduced to measure model performance versus the timescale of variation. The maximum entropy regularized spot models show the best agreement with solar observations reproducing the total sunspot area variation on time scales ranging from a few months to the activity cycle, although the model amplitudes are affected by systematic errors during the minimum and the maximum activity phases. The longitudinal distributions derived from the models compare well with the observed sunspot group distributions except during the minimum of activity, when faculae dominate the rotational modulation. The resolution in longitude attainable through the spot modelling is about 60 degrees., Comment: 21 pages, 7 figures, accepted by Astron. Astrophys

Multiband VRIJHK photometry of the Blazar PKS 0537-441 obtained with the REM telescope from December 2004 to March 2005 is presented. A major period of activity is found with more than four magnitudes variability in the V filter in 50 days and of 2.5 in 10 days. In intensity and duration the activity is similar to that of 1972 reported by Eggen (1973), but it is much better documented. No clear evidence of variability on time-scale of minutes is found. The spectral energy distribution is roughly described by a power-law, with the weaker state being the softer., Comment: 4 pages, 3 figures, A&A letters, in press

We present Doppler Images of the RS CVn binary system HR 1099 (V711 Tau) from spectra taken in two different sites, KPNO and MSO, during the MUSICO S 1998 campaign. Contemporaneous APT photometry is used to constrain the Doppler Images. The resulting maximum entropy reconstructions based on the least-square s deconvolved profiles, derived from $\sim$2000 photospheric absorption lines, r eveal the presence of starspots at medium-high latitudes. We have obtained maps for both components of the binary system for the first time. The predominant str ucture in the primary component is an off-centered polar spot, confirming previo us works on the same target by using independent codes. The result is verified b y using both data sets independently. The lower spectral resolution data set giv es a less detailed map for the MSO data set. The images obtained for the seconda ry component show a low latitude spot around orbital phase 0.7. This spot seems to mirror the structure seen on the primary. It might suggest that tidal forces may influence the spot distribution on this binary system., Comment: 12 pages, 9 figures. Accepted by A&A

Observations of the prompt afterglow of Gamma Ray Burst events are unanimously considered of paramount importance for GRB science and related cosmology. Such observations at NIR wavelengths are even more promising allowing one to monitor high-z Ly-alpha absorbed bursts as well as events occurring in dusty star-forming regions. In these pages we present REM (Rapid Eye Mount), a fully robotized fast slewing telescope equipped with a high throughput NIR (Z', J, H, K') camera dedicated to detecting the prompt IR afterglow. REM can discover objects at extremely high red-shift and trigger large telescopes to observe them. The REM telescope will simultaneously feed ROSS (REM Optical Slitless spectrograph) via a dichroic. ROSS will intensively monitor the prompt optical continuum of GRB afterglows. The synergy between REM-IR cam and ROSS makes REM a powerful observing tool for any kind of fast transient phenomena.

We discuss ROSAT, ASCA, {\it Beppo}SAX and optical observations of the young active star Gl 355. During the ROSAT observation a strong flare was detected with a peak flux more than an order of magnitude larger than the quiescent level. Spectral analysis of the data allows us to study the temperature and emission measure distribution, and the coronal metal abundance, for the quiescent phase and, in the case of ROSAT, also during the evolution of the flare. We have modeled the flare and derived a loop semi--length of the order of $\sim 1.5$ stellar radii. ROSAT, ASCA and {\it Beppo}SAX data suggest that the coronal abundance of Gl 355 is subsolar, in the range $0.1 \div 0.3 Z/Z_\odot$. A preliminary analysis of optical spectra allows us to compare the photospheric and coronal metal abundances., Comment: Proceeding for the "Stellar Coronae in the Chandra and XMM-Newton Era", 25-29 June 2001, ESTEC, Noordwijk, The Netherlands

We present multifrequency VLA and VLBA observations at 8.4 GHz of the RS CVn system AR Lac, that were performed in autumn 1997 simultaneously with X-ray observations obtained from Rodono` et al. (1999). Our VLBA data indicate a resolved source with dimension close to the system separation, while the study of the flux density curve evidences a small amplitude outside of the eclipse variability. The derived five-frequencies spectra, combined with the size information from VLBA data, are compared with gyrosynchrotron emission from a two component structured source. A comparison with the results of the X-ray observations allow us to exclude the possibility that thermal gyrosynchrotron is responsible for the radio emission, but it is compatible with the hypothesis of co-spatial X-ray and radio emitting sources., Comment: 9 pages, 7 figures. Accepted for publication in A&A