Abstract. We present the results from a spectroscopic Ca II H&K survey of 1058 late-type stars selected from a colorlimited subsample of the Hipparcos catalog. Out of these 1058 stars, 371 stars were found to show significant H&K emission, most of them previously unknown; 23% with strong emission, 36% with moderate emission, and 41% with weak emission. These spectra are used to determine absolute H&K emission-line fluxes, radial velocities, and equivalent widths of the luminosity-sensitive Sr II line at 4077Å. Red-wavelength spectroscopic and Strömgren y photometric follow-up observations of the 371 stars with H&K emission are used to additionally determine the absolute Hα-core flux, the lithium abundance from the Li I 6708Å equivalent width, the rotational velocity v sin i, the radial velocity, and the light variations and its periodicity. The latter is interpreted as the stellar rotation period due to an inhomogeneous surface brightness distribution. 156 stars were found with photometric periods between 0.29 and 64 days, 11 additional systems showed quasi-periodic variations possibly in excess of ≈50 days. Further 54 stars had variations but no unique period was found, and four stars were essentially constant. Altogether, 170 new variable stars were discovered. Additionally, we found 17 new SB1 (plus 16 new candidates) and 19 new SB2 systems, as well as one definite and two possible new SB3 systems. Finally, we present a list of 21 stars that we think are most suitable candidates for a detailed study with the Doppler-imaging technique.Key words: stars: activity -stars: chromospheres -stars: late-type -stars: rotation -surveys Tables A1-A3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html Visiting Astronomer, Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation. Scientific motivation for a Ca II H&K surveyThe presence of emission in the core of the Ca II H and K resonance lines is a diagnostic of magnetic activity in the chromospheres of late-type stars. Spatially resolved K-line heliograms and magnetograms amply demonstrate the relation between H&K-emission strength and the surface magnetic field on our Sun (Schrijver 1996). Furthermore, the fact that we observe generally stronger H&K emission in more rapidly rotating stars is widely known as the rotation-activity relation (e.g. Noyes et al. 1984) which is heuristically explained by the Ω-effect of the classic αΩ dynamo (see Stix 1989). Therefore, rapidly-rotating stars offer laboratories to study the effect of stellar dynamos. The catalog of chromospherically active binary stars (CABS, Strassmeier et al. 1993) summarized such stars in binaries and proofed to be a valuable data base for further investigations.It is only the very rapidly-rotating stars where we can also obtain spatially resolved information of their surface temperature distribution, and r...

Aims. We study the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations. Methods. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Results. Fifteen stars definitely show multiple cycles, but the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics and vary in parallel, indicating a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars. Conclusions. Stellar activity cycles are generally multiple and variable.

We describe the University of Vienna twin automatic photoelectric telescope (APT) located at the new Fairborn Observatory at Washington Camp in southern Arizona. Not only are the telescopes fully automatic, but the observatory itself is automatic. A site-control computer monitors weather sensors and operates the roof while the telescope control computer operates the photometer and accepts input files from and provides nightly observations to the astronomer; no direct operating personnel are needed. Both telescopes have 0.75-m primary mirrors, a CCD finder camera, and an environmentally controlled single-channel photoelectric photometer with filter combinations optimized for use in the blue (Wolfgang) and in the red (Amadeus) wavelength regions. The telescopes are currently used for monitoring spotted stars, for asteroseismology of nonradially pulsating variables, and for monitoring asymptotic-giant-branch stars. Some first results are presented.

Abstract. We reconstruct a time series of 28 surface temperature maps (Doppler-images) of the spotted single K2-dwarf LQ Hya from 35 consecutive stellar rotations in Nov.-Dec. 1996. Two more maps are obtained from data in late April and early May 2000. All maps show spot activity preferably at low latitudes between −20• and +50• , with a concentration in a band centered at around +30• , and with only occasional evidence for a higher-latitude spot extension. No trace of a polar spot is found at any of the above epochs. Most of this morphology can be reproduced by our flux-tube emergence model, except for the equatorial activity where the strong Coriolis force due to the rapid rotation always deflects flux tubes to higher latitudes. We also present the detection of weak differential surface rotation from a number of cross-correlation maps of the time-series images in late 1996. A solar-type differential rotation law, i.e. the equator rotating faster than the poles, with ∆Ω = +0.022 rad/day (lap time of ≈280 days) is in agreement with the data. Using the available photoelectric observations from 21 years we refine the rotation period to 1.60066 ± 0.00013 days and find a remarkable phase coherence over the course of 21 years, supporting the recent finding of active longitudes by Berdyugina et al. Furthermore, our photometry shows a complex multi-cyclic longterm brightness variability with three periods of 13.8 ± 2.8 years, its harmonic 6.9 ± 0.8 and 3.7 ± 0.3 years, respectively. The 3.7-year period would be in good agreement with the fundamental-mode oscillation period predicted by Kitchatinov et al. from a distributed-dynamo model, but remains to be confirmed.

Context. FK Comae Berenices is a rapidly rotating magnetically active star, the light curve of which is modulated by cool spots on its surface. It was the first star where the "flip-flop" phenomenon was discovered. Since then, flip-flops in the spot activity have been reported in many other stars. Follow-up studies with increasing length have shown, however, that the phenomenon is more complex than was thought right after its discovery. Aims. Therefore, it is of interest to perform a more thorough study of the evolution of the spot activity in FK Com. In this study, we analyse 15 years of photometric observations with two different time series analysis methods, with a special emphasis on detecting flip-flop type events from the data. Methods. We apply the continuous period search and carrier fit methods on long-term standard Johnson-Cousins V-observations from the years 1995−2010. The observations were carried out with two automated photometric telescopes, Phoenix-10 and Amadeus T7 located in Arizona. Results. We identify complex phase behaviour in 6 of the 15 analysed data segments. We identify five flip-flop events and two cases of phase jumps, where the phase shift is Δφ < 0.4. In addition we see two mergers of spot regions and two cases where the apparent phase shifts are caused by spot regions drifting with respect to each other. Furthermore we detect variations in the rotation period corresponding to a differential rotation coefficient of |k| > 0.031. Conclusions. The flip-flop cannot be interpreted as a single phenomenon, where the main activity jumps from one active longitude to another. In some of our cases the phase shifts can be explained by differential rotation: two spot regions move with different angular velocity and even pass each other. Comparison between the methods show that the carrier fit utility is better in retrieving slow evolution especially from a low amplitude light curve, while the continuous period search is more sensitive in case of rapid changes.

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Abstract. We present the first Doppler image of the single G0-dwarf HD 171488. As a 30-Myr young field star with a lithium abundance of ≈140-times that of the Sun and a rotation 20 times faster, it is hardly a "solar analog" but could be termed an "infant Sun". Its position in the H-R diagram suggests it to be in the rapid-braking phase just prior to arrival on the ZAMS. Our Doppler images from four spectral lines show a cool polar spot and various high-latitude spot features with a temperature contrast of 500-1600 K relative to the effective (photospheric) temperature. Low-to-medium latitude features may be present but appear to be biased by the uneven phase coverage of our spectra and are too weak to be conclusively judged significant. We determine the rotational period to be 1.3371 ± 0.0002 days and find a long-term, possibly cyclic variation of the mean V brightness of ≈7 yrs. A constant radial velocity of −23.6 ± 1.5 km s −1 suggests that HD 171488 is indeed a single star. Rising flux-tube models indicate preferred surfacing latitudes between 25• -60• for HD 171488 while our Doppler images reveal mostly high-latitude spots in excess of 60• . We emphasize that this discrepancy exists for all rapidly rotating single G dwarfs observed so far.

Abstract. We present continuous multicolor photometry for 47 stars from October 1996 through June 1997. Altogether, 7073 V (RI) c , UBV , and by data points, each the average of three individual readings, were acquired with three automatic photoelectric telescopes (APTs) at Fairborn Observatory in southern Arizona. Most of our targets are chromospherically active single and binary stars of spectral type G to K but there are also four pre-main-sequence objects and three pulsating stars in our sample. The light variability is generally due to rotational modulation of an asymmetrically spotted stellar surface and therefore precise rotational periods and their seasonal variations are determined from Fourier analysis. We also report on photometric variations of γ CrB (A0V) with a period of 0.44534 days. All data are available in numerical form.