Moffett (1972) pointed out the existence of short-time outbursts in flare stars as long ago as 1972. Gershberg & Petrov (1986) had registered a flare in EV Lac with a duration of 28.4 and an amplitude of 38.4 in 1984 with ASTRON. In recent years, a few reports appeared about observations of short-time outbursts in EV Lac with a duration in the range from 08.1 to several seconds. These results were summarized briefly by Zhilyaev (1993). Our observations confirm the existence of short-time flare activity on EV Lac.

The observations reported here were obtained at the 60 cm telescope at the Peak Terskol high-altitude Observatory (3 100 m above sea level) with a highspeed two-channel photometer (Zhilyaev et al. 1992) in B as well as simultaneously in both, U and V bands, with a time resolution down to 08.1. A new prefiltering technique (Zhilyaev 1995) has been used for the detection of smallscale activity of the flare star EV Lac.

The flare star EV Lac shows a wide variety of activity phenomena on a wide range of time scales. These are the well-known “classical” solar-like flares, the short-time flare events (Gershberg & Petrov 1986, Karapetian & Zalinian 1991, Tovmassian & Zalinian 1988, Tsvetkov et al. 1986, Zhilyaev 1994), and the long-duration variations caused by starspots and rotation (Petterson 1980). However, the “classical” flares are not such a frequent phenomenon as one gets used to think. As noted by Roizman & Kabitchev (1985), a considerable fraction of flares shows a complicated multipeak structure with sudden onset and decay. Their interpretation along the lines of classical models favored for solar-like flares is very problematic.

Here, we report the detection of high-frequency chaotic variability (flickering) in EV Lac. Optical flickering is observed in cataclysmic binaries, and in X-rays it is observed in low-mass X-ray binaries. As a rule, flickering is an accretion phenomenon. Strange as it may seem, EV Lac as a visual binary has the analogous property.

Spectroscopic and photometric monitoring of the flare star EV Lac was carried out with the 6-m telescope of the Special Astrophysical Observatory on the following 3 nights: 1993 September 10 (18:54-20:16 UT), 11 (17:46-20:03 and 21:25-22:16 UT) and 12 (19:46-20:08 and 21:23-22:21 UT).

We used the observation method described in detail by Berdyugin et al. (1994) and Alekseev et al. (1994). Observations were carried out with the 1024 channel TV scanner (Drabek et al. 1986) installed on the spectrograph SP-124 at the Nasmyth focus which covered a wavelength range 3480-5530Å with a spectral resolution 2Å per channel. Exposure times were 3 — 5 minutes, the signal/noise ratio was 20 at 3500Å and 40 at 5500Å and dead time between exposures was 10 — 15 sec. 92 spectra were obtained in total.

An investigation of the activity of the flare star AD Leo was made using U-band patrol observations. 60 cm computer-controlled telescopes with identical single channel photon-counting photometers at the National Astronomical Observatory at Rozhen and at the Belogradchik Observatory were used. The integration time was 1 sec. Differential photometry was carried out, the AD Leo measurements were made relative to BD+20°2475. The data was reduced with the program package APR (Kirov et al. 1991).

Two simultaneous observing runs were made. The results of the first have already been published (Antov etal. 1991). The other was part of the observational campaign in May, 1991 with the ROSAT satellite. In Bulgaria, the observations were carried out at Rozhen in the U-band and at Belogradchik in the B-band. Four flares were detected by both.

An analysis of optical, radio and X-ray observations of the flare of December 31, 1991 on UV Ceti is presented. Radio spikes at 4750 GHz are interpreted in terms of both ECM and plasma emission. Resulting parameters of the flare plasma are discussed.

During a search for flaring optical counterparts to γ-ray burst sources (GRBs) on archival patrol plates, an optical outburst image was found in the 3200 arcmin2 error box of the GRB which occurred on 1978 October 6 (GB 781006B = GBS 0008+13; Greiner et al. 1991) and was named S 10933 according to the convention of new Sonneberg variables. This outburst image appears on three simultaneously exposed plates taken on 1966 August 14/15 from 23.33 to 0.13 UT. The brightness estimates yielded mpg = 13m.3 ± 0m.1 and mv = 13m.8 ± 0m.2 with a high negative colour index of C = −0m.5 ± 0m.3.

During UBV photoelectric measurements of G102-21 - a dM3 star recently discovered as a remarkably active X-ray source by Micela et al. (1995) - we have observed the flare shown in Fig. 1 (left panel). The observed flux increase at flare maximum and the energy output (cf. Table 1) make this event one of the largest amplitude and most energetic flares ever detected on UV Cet-type stars.

The ratios EU/EB (=8.9) and EU/EV (=3.9) are more than 7 and 2 times larger, respectively, than the mean values found for typical dMe flares by Lacy et al. (1976). Moreover, assuming the empirical relation between the absolute magnitude MV and the mean energy dissipation during a flare (Gurzadyan 1980), the energy released in the U band exceeds the predicted one by a factor of ∼ 3 · 103. Clearly, we observed a rather peculiar and rare event that does not appear to be a typical flare on red-dwarf stars.

Recently, some apparently single red giants with evidence of chromospheric activity were reported. In this work we observed those single late giants with reported flares, referenced by Pettersen (1989) and Schaefer (1989), as part of a program for investigation of the chromospheric activity in G and K giants.

According to Simon & Drake (1989), active single G and K giants may be first crossing stars that have just entered the phase of rapid spin-up as their convection zones advance upon their more rapidly rotation radiative cores. These stars are situated on the base of the red giant branch on the HR diagram. As a result they may possess dynamo driven activity at this stage of evolution.

Stellar flares are one of the most challenging problems in astrophysics. Up to now, most of the detected flare stars are of late spectral type (e.g. UV Cet type stars etc.). Non-classical flare stars with early spectral type are very rare.

Recently, an A5-type flare star BD+47°819, near the open cluster α Persei, was discovered (Wang 1993). The flare U amplitude was 1m.5 in ten minutes. The V magnitude of the star was 9m.25 during the quiescent state.

On 1993 February 28 and March 1, spectroscopic observations of BD+47°819 were madewithn the 2.16 m telescope of Beijing Astronomical Observatory. A grating with a dispersion of 5nm/mm was used. Figs. 1 (a) and (b) show the observed spectra in the two nights. The exposure time for each spectrum was 15 minutes. Fig. 1 (a) shows a strange feature, an “emission line” at around 676 nm. Is this a real feature?

BY Dra has been extensively monitored for flares and spot-modulation of brightness at the Bulgarian National Astronomical Observatory (Panov & Ivanova 1993). Its mean light level exhibited remarkable changes in the past, due to the emergence and disintegration of photospheric spots, and different cycles have been proposed for the long-term changes of the BY Dra activity. This star is still believed to be a good candidate for cyclic variations, presumably similar to the sunspot activity cycle. After a spectacular drop in brightness in 1965 — 1967, and a subsequent rise in the early 70s, the mean light remained approximately constant during 1977 - 1991. A brightening of BY Dra occurred in 1992.

In Fig. 1, the light curves are shown for 1991, 1992 and 1993 in the V-band and in B — V. Phases were calculated with the ephemeris of Chugainov (1966):

JD (min. light) = 2438983.612 + 3.836 · E

From Fig. 1, it is apparent that the mean visual light, which was about V = 8.20 in 1991, increased in 1992 by 0m.10, and a further increase by another 0m.03 occurred in 1993. The phase of minimum light in 1992 indicates that the position of the spot remained approximately the same in stellar longitude as in 1991. However, in 1993 the position of the spot changed by almost half a period.

CN Com = CSV 6907 was dicovered by Romano (1958). First we note that the GCVS (and Simbad) coordinates are wrong by ≈2′. Using the finding chart of Romano (1958) and the APM data (digitized POSS), the correct coordinates of CN Com are R.A.(2000.0) = 12h19m47s.0, DEC(2000.0) = +16°30′50″.

CN Com is only 8″ distant from the ROSAT X-ray source RX J1219.7+1630 detected during the All-Sky-Survey at a PSPC countrate of 0.022 cts/s. Due to the positional proximity and the absence of other optical objects brighter than 20rmm within the about 30″ error circle, CN Com is very probably the optical counterpart of RX J1219.7+1630.

CN Com was investigated on some 600 photographic plates (Sonneberg astrographs 400/1600 mm and 400/2000 mm) of the fields 26 Com and 5 Com, covering the time interval from 1962 to 1994 (with only a very few plates from the years 1967-1974).

Photometric observations of V1054 Oph (Wolf 630 AB, mv = 9.69) and of the comparison star HD 152678 (mv = 8.1) were carried out in the UBV bands at Catania Astrophysical Observatory on 1994, June 15, using the 91 cm Cassegrain telescope equipped with an improved version of the twin-beam photometer URSULA (De Biase et al. 1988). We have detected a relatively low-intensity flare event preceded by a pre-flare dip of low amplitude and the exceptionally long duration of 36 min. The integration times were 10, 5 and 5 sec in the U, B and V bands, respectively. The variable and the comparison stars were observed simultaneously in the two independent channels of the twin-beam photometer. The sky background was subtracted from the photon count rates of V1054 Oph and its comparison star in each spectral band, and the flux of the variable was normalized to that of the comparison star.

Preliminary results of a photometric search for non-flare activity of the flare star EV Lac during June-August 1994 are presented. These observations are part of the long-term project entitled “Investigation of pre-flare processes on flare stars” being carried out at the Observatory of the Jordan Valley Regional College since July 1994. The project was motivated by reports of the existence of slow, low-amplitude non-periodic variability of both spectral lines and continuum outside/between flares (Rodono et al. 1979, Roizman et al. 1984).

The gas motion in close binary systems has been investigated by several authors using a hydrodynamic method (e.g. Prendergast & Taam 1974, Flannery 1975, Lubow & Shu 1975, 1976). All these researches have, besides important results, a common deficiency: in the analysis of the gas motion, the extent and velocity of the stream in the neighbourhood of the inner Lagrangian point L1 was not calculated. They have been chosen on the basis of general assumptions (the same for all types of close binaries). A fundamental assumption was the small extent of the stream in the neighbourhood of L1 perpendicular to the line connecting the centers of the stars. At the same time the extent and the velocity of the stream near L1 exert a great influence on the further stream motion, on the formation of the circumstellar envelope and finally on the evolution of the close binary system. Therefore the first step in the investigation of the gas motion in a close binary must be the determination of the initial parameters of the stream in the neighbourhood of the point L1.

Five UV Cet-type flare stars were photoelectrically monitored during the period 1970 to 1981 using either the Carl Zeiss Jena 1.04m reflector or the 0.56 m reflector of the Uttar Pradesh State Observatory Nainital. Thermoelectrically cooled EMI 6094S and 1P21 photomultipliers were used with the 1.04 m and 0.56 m telescopes, respectively. The sky brightness was also recorded frequently for short intervals during continuous monitoring of the stars.

After Haro’s fundamental discovery of flare stars in stellar associations and young clusters, their importance was fully recognized. The flare star system provides one of the most important records of the stellar aggregate’s history. This record can be used to establish the chronology of these systems and to test theories of star and aggregate formation.

Unfortunately, the observational material contains an unavoidable strong selection with respect to the statistics and the physical characteristics of flare stars in aggregates. This explains the difficulties which arise when one tries to compare physical and statistical characteristics of flare stars in aggregates and in the solar neighborhood, since the latter objects are studied almost exclusively by photoelectric methods. The flare frequency in aggregates is more than one order of magnitude smaller than that of flare stars in the solar neighbourhood. This is probably a selection effect, since the photographic observations in aggregates have been carried out with exposure times 5-10 min. Small amplitude flares cannot be recorded at all. Flare stars in aggregates are usually objects whose recorded flares have amplitudes > 0m.6 − 0m.7, and which last for at least 5 min. The long exposure masks the true amplitude of the flare.

To date we obtained 511 blue plates with multiple exposures of nine young clusters and associations to be used for studying the flare activity-age relation. First attempts showed already that automatic search increases the number of discovered flares by 50% compared to visual inspection (Aniol et al. 1989). Improved algorithms have now been implemented and applied. Automated object identification in existing catalogues (FSDB, GCVS, SIMBAD) is also available.

All plates are digitized with the PDS 2020 GMplus microdensitometers at AIM and reduced with the AIM software. Steps following the reconstruction of images include the transformation of plate to sky coordinates and matching of the multiple exposure chains with the stars on the deep reference plate. All complete chains (generally six exposures) are used to derive average light curves for all objects. For each magnitude interval 10-11, 11-12, … 15-16 the mean deviation and standard error of an image from the overall mean value of the respective chains is determined. If any image in a chain differs by more than a preset multiple of the mean error it is considered a potential flare event and registered for visual inspection, together with the chains immediately preceding and following the suspected flare event, to obtain information on the total duration of the outburst. The candidates are filed and the data of all stars are stored for checks of long-term variability.

An updated data collection for UV Cet type stars and their registered flare events in stellar aggregates and in the solar neighbourhood during more than 9580 hours observing time is described. The Flare Stars Database (FSDB) presently includes 3127 entries for 1622 stars.

At Los Alamos National Laboratory we are developing a new imaging sensor which combines high spatial and high temporal resolution over a large area format, while maintaining single-photon counting sensitivity and sustaining a high count rate. The detector is called a microchannel plate with crossed delay line readout, or MCP/CDL. This detector is ideally suited to the observation of weak transient events, such as stellar flares from red dwarf flare stars in our Galaxy. At present we are initiating an experiment with the MCP/CDL detector which will utilize a 30-cm aperture f/7 telescope to characterize U-band, B-band, and V-band emission from such low-luminosity flare stars, and to search for weak optical transients associated with other astrophysical sources.

Instrumentation and observing methods are developed for optical high-speed astrophysics, aiming at exploring milli-, micro-, and nanosecond variability. Such rapid fluctuations can be expected from instabilities in accretion flows, and in the fine structure of photon emission. For the optical, we have constructed a dedicated instrument, whose first version was tested on La Palma to study atmospheric scintillation on very short timescales. A second version is now under development, using photon-counting avalanche photodiodes as detectors.