The radio emission of quiescent active regions at 6 cm wavelenght marks the legs of magnetic dipoles, and the emission at 20 cm wavelength delineates the radio wavelength counterpart of the coronal loops previously detected at X-ray wavelengths. At both wavelengths the temperatures have coronal values of a few million degress. The polarization of the radio emission specifies the structure and strength of the coronal magnetic field (H 600 Gauss at the heigths h 4 x 109 cm above sunspot umbrae). At 6 cm and 20 cm wavelength the solar bursts have angular sizes between 5" and 30" brightness temperatures between 2 x 107 K and 2 x 108 K, and degrees of circular polarization between 10% and 90%. The location of the burst energy release is specified with second-of-acr accuracy. At redio wavelength the burst occur within the central regions of magnetic loops, while the flaring Hα kernels are located at the loop footpoints. Coronal loops exhibit enhanced radio emission (preburst heating) a few minutes before the release of burst energy. The radio polarization data indicate magnetic changes before and during solar bursts.

It has been known for many years that the flare build-up manifests at centimeter wavelengths (2-6 cm), in the form of increased intensity and increased polarization of the active region. The flare-associated bursts originate in these intense sources, and the probability of occurrence of bursts increases with the increasing intensity of these narrow bright regions. With the availability of arc-second resolution using the VLA it has been possible to study the nature of this build-up from two-dimensional synthesized maps over short periods before the start of a flare. For a hard x-ray associated impulsive 6 cm burst observed on June 25, 1980 (Kundu, Schmahl, and Velusamy 1981), we produced several 15-minute synthesized maps in total intensity (I) and polarization (V) just before the flare onset (Kundu 1981). Figure 1 shows the central 11x16 regions of 15 minute synthesis maps over the period 14:45-15:45 UT. As can be seen from these Figures, the region is very complex, consisting of numerous components many of which are bipolar. These components have brightness temperatures of 6-9x106 K during the hour before the flare. The burst source was located close to the neutral line of these oppositely polarized regions near B. The burst maximum is identified, with a “+” and the burst extent averaged over the period 1551-1600 UT is shown by the dotted contour in the last map.

The aim of the VLBI experiment was a search for the occurrence of subarc- second microwave emission centres as tracers of the initial energy release in solar flares. The observations extended over the period April 28 to May 3, 1981 and were performed with the 25 m telescopes at Onsala (Sweden) and Dwingeloo (Netherlands) at a wavelength of 18 cm (1663 MHz). The baseline was 619 km long giving a minimum angular width of the interference fringes of 0.06’. This corresponds to a spatial scale of 40 km at the solar disc centre.

This review presents a summary of observed photospheric phenomena on RS CVn stars: the amplitude, shape, evolution and migration rate of the photometric wave in relation to the rotational and orbital motion.

The main points considered are: 1) the activity level (maximum amplitude, short and long timescale variability) versus rotation period; 2) the activity cycles as inferred from changes in the wave migration rate and direction and from the variation of its amplitude; 3) the detection of differential rotation; 4) the connection between the orbital period variation and activity.

The observational information regarding chromospheric emission features in surface-active RS CVn stars is reviewed. Three optical features are considered in detail: Call H and K, Balmer H-alpha and Hel λ 10830. While the qualitative behavior of these lines is in accord with solar-analogy/rotation-activity ideas, the quantitative variation and scaling is very poorly understood. In many cases, the spectroscopic observations with sufficient S/N and resolution to decide these questions have simply not yet been made. The FK Com stars, in particular, present us with extreme examples of rotation that may well press our understanding of surface activity to its limits.

We discuss a newly-developed technique for spatially resolving starspots on some of the more rapidly rotating RS CVn stars. Basically, the method uses high resolution, very high signal-to-noise spectral line profiles and exploits the Doppler velocity correspondence between position across the stellar disk and wavelength position across a rotationally broadened line profile to synthesize an image of the star, showing the location, sizes, and shapes of its starspots. Though still in a developmental stage, the technique is already yielding information about the structure and general appearance of starspots. Examples of Doppler Imaging observations of HR 1099 will be presented, along with a movie showing the behavior of synthetic line profiles generated from a computer spot model.

We present a complete set of yearly seasonal light curves (LC) of RS CVn obtained from 1963 to 1982 at Catania Observatory, integrated with two LCs obtained by Ludington (1978) in 1975 and 1976. This unique observation set vividly shows (Figure l) the migrating outside of eclipse “photometric wave” (PW), which was first evidenced by Catalano and Rodonò (1967) and it is now considered the distinctive photometric feature of RS CVn binaries as well as of other spotted stars. By attributing the PW to surface inhomogeneities, or spots, whose visibility is modulated by the spotted star rotation, the present PW migration toward decreasing orbital phases indicates that the angular rotation of the KOIV spotted component is slightly lower than the orbital one. An inspection of Figure 1 readly shows that the almost sinusoidal PW has been fairly stable from 1963 up to 1981, when a double-peaked wave, still present in the 1982 LC, developed. Most probably the “unusual” 1949 LC by Keller and Limber (1951) and the 1976 one by Ludington (1978) were obtained at a similar activity phase.

Our study of the period distribution of about 200 FGKM-type unevolved close binaries has revealed a strong deficit of short-period systems.This finding may be connected with the occurrence of a very efficient mechanism of orbital angular momentum loss via magnetic braking by stellar wind, in the earliest evolutionary phases.

One of the properties of RS CVn-like binaries is the presence of cyclic fluctuations, sometimes called “migrating waves”, in the V and other broad-band lightcurves. These fluctuations, perhaps due to spots, vary in amplitudes and periods, in the sense that, the longer the orbital period of the RS CVn system, the higher the amplitude of the “wave”, which may be up to 30-35% of the total light variation. Therefore, in short-period (less than one day) RS CVn binaries, these fluctuations are generally difficult to be detected.

One of the outstanding features of RS CVn-type binaries is the changing shape of the so called “wave-like distortion” or “photometric wave” (PW) and its phase migration on the light curve (LC). Here we present a preliminary analysis of the observations of the following objects: CQ Aur, RU Cnc, VV Mon and SZ Psc. For each system we have derived the amplitude and phase of the PW maximum and its migration rate.

The eclipsing binary WY Cancri (G5 V, M2 V), having a period of od. 82937122, has been classified as a short period RS CVn type binary (Hall, 1976). Chambliss (1965), Oliver (1976, Sarma (1976), Awadulla and Budding (1979) have reported observations of this system. In order to determine a more reliable period of the distortion wave, its source, nature and period of the overall luminosity change from one observing season to the next, we have included this star in the observing schedule of RS CVn binaries at our observatory. Observations were made in standard UBV filters using the ^48-inch reflector telescope of the Japal-Rangapur Observatory. The stars BD+28° 1672 and BD+27° 1701 were used as a comparison and check stars respectively.

AR Lac is an eclipsing binary of the RS CVn type. The intrinsic variability of one of the components was first announced by Wood (1946) and the presence of sharp H and K emission lines was noted by Wyse (1934). More recent work by Nha and Kang (1982) shows clearly that the light curve varies from year to year. The period is variable.

HK Lac was observed at Konkoly Observatory between 1976-1981, the details of its story have been described in a previous paper (Olah, 1979).

The aim of this investigation is to give a possible description of the strong variations in the light curve of the star.

Searching for a more accurate period, it became obvious that the same period was not applicable to all the data. Therefore, a new period was derived for each different cycle by the Lafler-Kinman method. Unfortunately, due to the small number of data in the data sets these newly derived periods have an error of about 0.1 day.

AI Phe was discovered to be variable and identified as an EA binary by Strohmeier (1972). Reipurth (1978) subsequently carried out uvby photometry, determined the period (24.5923 d), and noted the lengthy totality of primary minimum and the displacement of the secondary minimum. Imbert (1978) obtained radial velocity curves and determined spectroscopic orbital elements. Imbert also gives a spectroscopic classification of G2V for the primary (hotter) component and approximately G5 for the secondary. AI Phe thus appeared to offer a unique opportunity to study the limb darkening of a non-interacting solar-type star. As an extension of a solar UV limb darkening study (Kjeldseth Moe and Milone 1978), ten IUE spectra were obtained on Aug. 12 in 2 successive shifts (cf Milone et al 1981). In conjunction with this, ground-based 5-colour photometry and spectroscopy were carried out at CTIO and at UTLCO.

The light variation of V711 Tauri was first discovered by Cousins, who suspected the variation with an amplitude of ΔV = 0.mll. This light variation was confirmed by Landis and Hall (1976). This star has been identified as the brightest RS CVn-type star by Bopp and Fekel (1976). Bopp et al. (1977), using Cousins' old data and their data made nearly 13 years later than those of Cousins, found that the observations show the same light curve shape and amplitude and the minimum light falls very nearly at 0.P0 computed with their ephemeris.

XY UMa (+55°1317, SA027143) is a short-period (P 0.48 d) cousin of the RS CVn stars. The primary star is G2-G5V; the secondary K5 (Geyer, quoted by Lorenzi and Scaltriti, 1977). Geyer (1977) has done the bulk of the observational work to date, including the first photoelectric observations. The rapid, annual changes in XY UMa's light curve, and the fact that the last published light curves was from the 1977 season convinced us to reobserve this active system.

The discovery of X-ray emission from RS CVn systems by HEAO-1 and subsequent surveys by the Einstein Observatory have shown that these close binaries exhibit greatly enhanced coronal activity. Here we review the 3 main observational areas: (1) results of the X-ray surveys of RS CVn systems and other late-type stars which indicate how the X-ray luminosity is correlated with the binary period (and hence stellar rotation) and other coronal activity indicators. This will be discussed in the context of scaled models of the solar corona; (2) X-ray spectroscopy of the most active systems which show multitemperature spectra and line emission consistent with solar abundances of the heavy elements; (3) observations of X-ray “flare-type” activity that has been associated with several RS CVn systems.

This is a summary of the systematic program of A3cm radio observations of RS CVn and similar binaries which has been carried out since 1977 at the Algonquin Radio Observatory. The observations were made at X-band frequencies (10-11 GHz) with the 46m telescope using the same observing procedures described in Feldman et al. (1978). The detection limit is generally 25 mJy, although it is possible to do somewhat better than this if the star is located well off the galactic plane and if it maintains an elevated flux level (i.e. ≳ 15 mJy) for several hours. The observing list has grown roughly four-fold since the start of the program in 1977. It now encompasses 65 of the 69 “bona fide” RS CVn binaries compiled by Hall (1982); the rest are suspected RS CVn binaries, BY Dra variables, Algol-type binaries, and W UMa binaries. To date, a total of some 85 observing days, representing more than 1000 hours of telescope time, has been devoted to this program.

The best way to learn about the detailed physical conditions associated with stellar activity is to construct detailed self-consistent model stellar atmospheres based on the wide range of diagnostics now available. A quicker rough analysis of the activity can be accomplished by studying the interrelations of the various diagnostics. Tight correlations between diagnostics suggest physical association, and the power law indices of the correlation can help to elucidate the general physical relation the diagnostics have with each other. Our aim in this study was to observe with the IUE satellite a homogeneous sample of stars for which there is extensive coronal data. The X-ray studies of close binaries by Walter and Bowyer (1981) and Walter (1982) provided such a sample; these stars are also accessible to the IUE because of their tendency to strong activity. Because of intrinsic stellar variability, this type of study is best carried out when all diagnostics are measured simultaneously. We have come fairly close to this ideal; only the coronal data is non-contemporaneous for most of the sample. Our reduction of the IUE data is quite similar to that of Ayres, Marstad and Linsky (1981, hereafter AML) and we have used essentially the same diagnostics.

The characteristic optical light curves of RS CVn - and BY Dra-type variables are believed to represent non-uniform distribution of dark spots akin to sunspots which are revealed by rotation. By analogy with the Sun, strong magnetic fields probably underlie this phenomenon, extending upwards into the chromosphere and corona, enhancing the emission from regions that overlie the spots. Previous work on the BY Dra variable AU Mic (Ref.l) did not clarify whether the fluxes from chromospheric and transition region lines were rotationally modulated in the sense that the phase of maximum emission was in register with spot visibility or minimum light. This important question prompted the need for further collaborative IUE, optical and radio work.