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Astron. Astrophys. 334, 931-934 (1998)

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2. Observations

We present EV Lac photometric observations carried out at three observatories in Ukraine, Russia and Bulgaria. The 60-cm Cassegrain telescope with a high-speed two-channel photometer (Zhilyaev et al. 1992) was used for all the data obtained at the Peak Terskol High-Altitude Observatory (North Caucasus, elevation 3100 m). The photometer has two independent synchronous channels with a photon counting system. Two operating modes are available. In the first mode two stars can be observed simultaneously. In the second one a single star can be observed in both channels by using the beam-splitter. Both operating modes were used for the photometry of EV Lac in the UBV bands, with a time resolution down to 0.05 s. Regular observations of EV Lac revealed the existence of a flare fine structure detected for first time on 1990 August 27 (Fig. 1). To refine our understanding of this phenomenon series of coordinated observations of EV Lac were carried out in Bulgaria, at Peak Terskol and in Crimea in 1995-96.

[FIGURE] Fig. 1. EV Lac on 1990 Aug. 27, 00:45 UT. U band (top), V band (bottom). Peak Terskol. The bar indicates the rms deviation for the quiet state

In Bulgaria the photometric observations were carried out at the Belogradchik Observatory, equipped with a 60-cm Cassegrain telescope and a single-channel photon-counting photometer (Antov and Konstantinova-Antova, 1995). The instrument operates in a semi-automatic mode. UBV and uvby photometric systems are available. The integration time used for EV Lac observations was 0.1 s. The monitoring was done synchronously with the 60 cm telescope at Peak Terskol. The time synchronization accuracy was of 1 s. The distance between both observatories is approximately 1500 km.

At the Crimean Astrophysical Observatory the photometric monitoring was carried out with a time resolution of 0.1 s using the 1.25 m reflector AZT-11 equipped with the UBVRI photometer-polarimeter described by Piirola (1984) and Kalmin & Shakhovskoy (1995). The sky background was subtracted from all the data presented here. The UBVRI data were converted in a magnitude scale with respect to the mean values of quiescent fluxes.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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