ИСТИНА

KV UMa is an X-ray nova consisting of an accreting compact object -- a black hole with a mass of ~ 7 M_Sun. Nova KT Eri consists of a white dwarf with a mass of ~ 1.2 M_Sun. Both objects have a red donor star in a late evolutionary stage. We made photometric observations of KV UMa during its outburst in January 2005, with the amplitude of the outburst of ~ 7 mag. Note that another registered outburst of this star occurred in 2000 yr. The outburst of KT Eri began in November 2009, we started the observations in January 2010, the total amplitude of the outburst was ~ 9 mag. Currently, no other outbursts have been registered. However, the light curves comparison of KT Eri with the analogous light curves of other nova stars and X-ray sources allows different authors and us to conclude that this star is a possible recurrent nova. These objects were also detected as X-ray sources: for KV UMa the X-ray peak began just after the optical maximum of the outburst; KT Eri was observed as a supersoft X-ray source (SSS) during the decline. The orbital period of KV UMa was confidently determined, showing a two-peak wave with an amplitude of up to 0.8 mag in quiescence and a single wave with an amplitude of a few hundredths of a magnitude, maintaining the same period. In contrast, for KT Eri different authors found different periods – from 0.09 to 730 days. For periodicity search over an extended time interval for KT Eri we obtained long series of CCD frames during its post-outburst quiescence, primarily from 2018 to 2023 yrs. Finally, we found slow brightness variations on time scales of a few months and longer, with the most probable value of period of ~450 days. Additionally, we identified short-period variability with a possible period of 0.186 days and an amplitude of ~0.06 mag, which appears at different magnitude levels of the system. This short period may correspond to the rotation period of the white dwarf. Recently for KT Eri B.E. Schaefer et al. from spectroscopic observations obtained wave of ~2.616 days, which they interpreted as the orbital period. During some separate intervals, this value of period appears both in Schaefer’s and in our photometric data series with a slightly variable amplitude of magnitude on the light curves. Besides the described variabilities, KT Eri exhibits strong flickering on timescales of a few dozen minutes and an amplitude up to 0.7 mag. We also analyzed the color variations at different activity stages. For both stars, the mean color temperature varies from 8000 to 11000 K on two-color diagrams (U-B, B-V), (B-V, V-R), during some stages of outburst activity, the position of KT Eri is situated above the blackbody line. The "color-absolute magnitude" diagram (i.e. HR-diagram) exhibits the location of KT Eri significantly higher than the white dwarf branch. This fact suggest that accretion plays an important role in the system, with the accretion disk contributing significantly to the total luminosity both during the outburst and in quiescence.