Vojkhanskaya & Mitrofanov (1980) found circular polarization up to 2 per cent, and supposed that MV Lyr could be a polar. However further study did not confirm this classification (Efimov & Shakhovskoi 1980, Robinson et al. 1981). Ultraviolet observations made by Chiappetti at al. (1982) are consistent with the existence of an accretion disk at different brightness states. Perhaps MV Lyr could be an intermediate polar (Warner 1983).
From spectroscopic observations in the low state Schneider et al. (1981) found the orbital period to be . In the high state Skillman et al. (1995) improved it to . They suggested that the binary consists of a main-sequence red dwarf of and a white dwarf of with a low binary inclination ().
The question of the photometric period is puzzling. All attempts to find a stable period were unsuccessful until Borisov (1992) and later Skillman et al. (1995) found periodic light variations during the high state. They interpreted them as superhumps with period 0.138 d, or 0.005 d longer the orbital period. MV Lyr was claimed to be a "permanent superhumper" during its high state, similar to the SU UMa-type dwarf novae (Warner 1985), which commonly show superhumps during their long outbursts (superoutbursts).
During the low state (outside of the brief outbursts) the light curve is sometimes very quiet, and Robinson et al. (1981) even suggested that the low state is caused by total cessation of mass transfer from the late-type companion. However, Andronov & Shugarov (1983), based on photographic observation during 5 nights, found "quasi-orbital" light variations with amplitude . In the next low state Pavlenko (1998a) confirmed the existence of such variations with typical timescale 0.13-0.14 d, but with amplitude less then , and also found low-amplitude light variations on a timescale d (Pavlenko 1998b), which might be connected with the precession period of the accretion disk.
Several photometric campaigns have been undertaken in the high state, but none so far in the low state, especially emphasizing the rapid variability occurring there. Here we present our photometric studies during an enormous outburst which occurred during the low state of 1997.
Observations were carried out in June-October 1997 at the 60-cm telescope (Zeiss-600) of the Crimean Laboratory of the Sternberg Astronomical Institute with an electrophotometer, and at the tv complex of the 50-cm telescope (MTM-500) of the Crimean Astrophysical Observatory, equipped with a high sensitive tv tube (Abramenko et al. 1988, Castro-Tirado et al. 1993).
A total of brightness measurements were obtained on 11 nights of observation: during JD 615-625 at the Zeiss-600 and during JD 624-713 at the MTM-500. During JD 624-625 the observations at two telescopes were overlapped in order to check the difference in color systems. The total exposure was 26 hours. Several brightness estimates were also made before and after these dates, during JD 321-614 and after JD 850.
All observations were done in a system close to a standard B. The typical time resolution was 10-20 s at the 60-cm telescope and 40-50 s at 50-cm telescope, for an accuracy of 1-2 percent. Several comparison stars were used: the star N1 for observations at Zeiss-600 (the designation is given in Andronov & Shugarov 1982) and stars N8 and N5 (from Rosino at al. 1993) for observations at MTM-500. After reduction to the B system the zero-points of the data sets coincided within , so no later correction was used.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999