Aims. We report results of a quantitative colorimetric UBVRI analysis of two flare events on the red dwarf EV Lac. The photometric data were obtained in September 2004, during the multi-site synchronous monitoring from the four observatories in Ukraine, Russia, Greece, and Bulgaria. These observations confirmed the presence of small-scale high-frequency oscillations (HFO) initially detected by Rodonó (1974, A&A, 32, 337) and recently reconfirmed by the authors. Here we discuss the color characteristics of flares and HFO. Methods. Colorimetric analysis had been performed with the help of the time tracks in the UBVRI color-color diagrams from the earliest phase of flare development. Digital filtering technique was used to evaluate the time-dependent color indices. Results. As can be clearly seen in the diagrams, color indices oscillate on a time scale of seconds, far exceeding instrumental errors. Regarding the HFO, we conclude that the bulk of a flare oscillates during a major part of its lifetime between the states of hydrogen plasma opaque and transparent in the Balmer continuum. We find that at the peaks of oscillations the color tracks drift into the regions of color-color diagrams corresponding to a blackbody radiation, which provides an estimate of color temperatures from 17 000 to 22 000 K. We also find that flares cover ∼1% of the stellar disc.

A new approach for the search of microvariability and high-speed phenomena in variable stars is presented. It gives a new technology of observation and analysis of the data. Statistical photometry stands on the three pillars: the digital filtering technique, the photon counting statistics based on the Mandel equation, and the integral transforms of the light curves. This technique permits to investigate low-amplitude and rapid variability of stars.

We have photometrically monitored the V p 8 mag Galactic Population I WC5 star WR 111 for 3 weeks nonstop using the MOST microsatellite. Each of the ∼27,000 data points has a precision of ∼3 mmag. We find no coherent Fourier components above the 50 part per million level over the whole interval for frequencies cd (periods P ! 2.4 hr). This limit is nearly 2 orders of magnitude below recent predictions for early-Ϫ1 f 1 10 type WR stars based on strange-mode pulsation simulations, with expected periods in the range 10-30 minutes. Simultaneous spectroscopic observations of WR 111 reveal a normal level of stochastic clumps propagating in the wind, which possibly manifest themselves in the slow rise in the MOST power spectrum below 1/f f ∼ 10 cd . Time-frequency analysis of the MOST data shows no obvious short-lived frequencies above the 1 mmag Ϫ1 level, in stark contrast to the highly variable cool WR stars WR 123 (WN8) and WR 103 (WC9d), monitored previously by MOST. Radiation pressure therefore appears to be the main, if not sole, driver of WR 111's strong wind.

Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m 2 of light gathering area, and distributed over ~1km 2 . These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.

Abstract. V390 Aur is classified as a G8 spectral class active single giant by Fekel and Marshall (1991). Multi-site cooperative high-speed photoelectric monitoring was carried out for it in the period 1998-2003 as part of a program developed in Bulgaria recently to study chromospherically active evolved stars. Astronomers from Greece and Ukraine joined the flare activity study of V390 Aur in 2000. As a result, several flares with a duration from seconds to minutes were detected on this star at 3 observatories. Taking into account all the events detected, the flare rate of the star is estimated to 0.205 flares per hour. Comparison with the active dwarfs AD Leo and EV Lac indicates that the flare rate of V390 Aur is lower than that of AD Leo but comparable with the well-known flare star EV Lac.

Abstract.A program of Many-sites Multi Channel simultaneous observations of flare stars by a network of telescops was initiated in 1998. The purpose of this program is to investigate the fine structure of the flares light-curve in UBVRI, depressing atmospheric and instrumental noises. In this paper we present the results of the observations on the Flare star EV Lac in the B color, which were carried out at the Stephanion observatory during the campaign of 1999. Discrete Fourier Transform analysis of the B-light curve of the largest flares reveal the presence of high frequency small amplitude oscillations with periods ranging between 6.9 sec and 20 sec in around the flare maximum phase. The oscillation amplitude ranges from 1.4 to 2.6 % of the star brightness in accordance with the results of the campaign of 1998 (Zhilyaev et al. 2000). In addition there is a tendency the higher frequencies to persist in the late phases of the flare evolution. The observations are consistent with the phenomenology of the evolution of a fast mode magneto-acoustic wave generated at the impulsive phase of the flare and traveling through the magnetic loop.

In this work we present the results of an investigation aimed at a search for an oscillatory phenomenon during short gamma-ray bursts. The wavelet technique, used for this analysis, is applied to the data from the BATSE 3B catalogue. We have detected oscillations, which periods are found to be in the milliseconds range and their amplitudes up to dozens of percents. A possible scenario for such a phenomenon is the coalescence of stellar-mass black holes and neutron stars. During the coalescence process the matter orbiting the black hole produces rapid, periodic phenomena. Such system will also emit gravitational waves which cause the orbital radius to decrease and leads to the emission of a chirp of radiation. Estimates lead to a timescale of milliseconds for the coalescence process and oscillation frequencies of hundreds of Hz. The gamma-ray bursts considered in this paper, show both frequencies and survival times of oscillations close to the mentioned values. A chirp phenomenon is also present. We therefore argue in favor of the black hole -neutron star coalescence as a scenario for the production of short gamma-ray bursts.

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