Nikolay Shakura∗1,2, Dmitry Kolesnikov1, Konstantin Postnov1,2, Igor Volkov1,3, Ilfan Bikmaev2, Tatiana Irsmambetova1, Rüdiger Staubert4, Joern Wilms5, Eldar Irtuganov2, Pavel Shurygin2, Polina Golysheva1, Sergey Shugarov1,6, Igor Nikolenko3, Eugene Trunkovsky1, Gabriele Schönherr8, Axel Schwope8, Dmitry Klochkov4 1Sternberg Astronomical Institute, Moscow State University, 119234, Moscow, Russia 2Kazan Federal University, Kazan, Russia 3Institute of Astronomy RAS, Moscow, Russia 4Institute for Astronomy and Astrophysics, Tubingen, Germany, 5Astronomical Institute of the University of Erlangen-Nuremberg, Bamberg, Germany, 6Astronomical Institute of the Slovak Academy of Scienses, Tatranska Lomnica, Slovakia 7Crimean Astrophysical Observatory, Nauchny, Russia 8Leibniz Institute for Astrophysics, Potsdam, Germany E-mail: nikolai.shakura@gmail.com, kolesnikovkda@gmail.com

The X-ray binary Her X-1 consists of an accreting neutron star and the optical component HZ Her. The 35-day X-ray variability of this system is known since its discovery in 1972 by the UHURU satellite and is believed to be caused by forced precession of the warped accretion disk tilted to the orbital plane. We argue that the observed features of the optical variability of HZ Her can be explained by free precession of the neutron star with a period close to that of the forced disk precession. The model parameters include a) the intensity (power) of the stream of matter flowing out of the optical star; b) the X-ray luminosity of the neutron star; c) the optical flux of the accretion disk; d) the X-ray irradiation pattern on the donor star; e) the tilt of the inner and outer edge of the accretion disk. A possible synchronization mechanism based on the coupling between the neutron star free precession and the dynamical action of non-stationary gas streams is discussed.

Multifrequency observations of the classical CO nova V339 Del, detected in all frequencies from $\gamma$-rays to radio, are reviewed. Nova V339 Del was discovered on 2013 August 14.584 UT. The times of decline from the brightness maximum $V$ = 4.4 mag, reached on 2013 August 16.47, UT, was estimated as $t_{2,V}$ = 10 days, $t_{3,V}$ = 18 days, so V339 Del can be classified as a fast nova with super-Eddington luminosity at maximum. The maximum-magnitude-rate-of-decline relations were used to determine an absolute magnitude at maximum $M_{V,max} = -8.70\pm$ 0.03, $M_{B,max}$ = -8.45 ${\pm}$0.08 and distance $d$ = 3.2$\pm$0.3 kpc, using the interstellar extinction $E(B-V)$ = 0.184${\pm}$0.035. The distance to the nova found by different methods is in the range 2.7 - 4.54 kpc. The white dwarf mass was estimated from $M_{B,max}$ as $M_{wd}$ = 1.04$\pm$0.02 M$_{\odot}$. We suggest that the eruption occurred on the surface of a CO white dwarf. V339 Del is the first nova that has been observed to synthesize the element lithium. The dust consisted of amorphous carbon grains was detected in infrared region. The $UBVR_{C}I_{C}$ light curves of nova constructed from daily means of all available data, including our own observations obtained from its discovery till August 2015, are presented and used to show the track of the nova in the color-color diagram during the first 100 days. The dust particles formed in the dense clumps of ejecta were responsible for the $U$ light curve variability. Our medium-resolution spectrum of V339 Del taken in August 2015 by the 6m SAO telescope in North Caucasus confirms a non-spherical structure of the ejected shell.