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We report the results of spectroscopic and photometric observations of the emission-line object AS 386. For the first time, we found that it exhibits the B[e] phenomenon and fits the definition of an FS CMa type object. The optical spectrum shows the presence of a B-type star with the following properties: T_ eff = 11000+/-500 K, log L/L_sun = 3.7+/-0.3, a mass of 7+/-1 M_sun, and a distance D = 2.4+/-0.3 kpc from the Sun. We detected regular radial velocity variations of both absorption and emission lines with the following orbital parameters: P_orb = 131.27+/-0.09 days, semi-amplitude K_1 = 51.7+/-3.0 km/s, systemic radial velocity gamma = -31.8+/-2.6 km/s, and a mass function of f(m) = 1.9+/-0.3 M_sun. AS 386 exhibits irregular variations of the optical brightness (V=10.92+/-0.05 mag), while the near-IR brightness varies up to ~0.3 mag following the spectroscopic period. We explain this behavior by a variable illumination of the dusty disk inner rim by the B-type component. Doppler tomography based on the orbital variations of emission-line profiles shows that the material is distributed near the B-type component and in a circumbinary disk. We conclude that the system has undergone a strong mass transfer that created the circumstellar material and increased the B-type component mass. The absence of any traces of a secondary component, whose mass should be >= 7 M_sun, suggests that it is most likely a black hole.

We perform high resolution direct $N$-body simulations to study the effect of an accretion disc on stellar dynamics in an active galactic nucleus (AGN). We show that the interaction of the nuclear stellar cluster (NSC) with the gaseous disc (AD) leads to formation of a stellar disc in the central part of the NSC. The accretion of stars from the stellar disc onto the super-massive black hole is balanced by the capture of stars from the NSC into the stellar disc, yielding a stationary density profile. We derive the migration time through the AD to be 3\% of the half-mass relaxation time of the NSC. The mass and size of the stellar disc are 0.7\% of the mass and 5\% of the influence radius of the super-massive black hole. An AD lifetime shorter than the migration time would result in a less massive nuclear stellar disc. The detection of such a stellar disc could point to past activity of the hosting galactic nucleus.

We observed RZ LMi, which is renowned for the extremely (~19d) short supercycle and is a member of a small, unusual class of cataclysmic variables called ER UMa-type dwarf novae, in 2013 and 2016. In 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, 60d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the novalike state (permanent superhumper). This observed behavior extremely well reproduced the prediction of the thermal-tidal instability model. We detected a precursor in the 2016 superoutburst and detected growing (stage A) superhumps with a mean period of 0.0602(1)d in 2016 and in 2013. Combined with the period of superhumps immediately after the superoutburst, the mass ratio is not as small as in WZ Sge-type dwarf novae, having orbital periods similar to RZ LMi. By using least absolute shrinkage and selection operator (Lasso) two-dimensional power spectra, we detected possible negative superhumps with a period of 0.05710(1)d. We estimated the orbital period of 0.05792d, which suggests a mass ratio of 0.105(5). This relatively large mass ratio is even above ordinary SU UMa-type dwarf novae, and it is also possible that the exceptionally high mass-transfer rate in RZ LMi may be a result of a stripped core evolved secondary which are evolving toward an AM CVn-type object.

We report the results of a long-term spectroscopic monitoring of the FS\u2009CMa type object MWC\u2009728. We found that it is a binary system with a B5 Ve (T$_{\rm eff}$ = 14000$\pm$1000 K) primary and a G8 III type (T$_{\rm eff} \sim$ 5000 K) secondary. Absorption line positions of the secondary vary with a semi-amplitude of $\sim$20 km/s and a period of 27.5 days. The system's mass function is 2.3$\times10^{-2}$ M$_\odot$, and its orbital plane is $13^{\circ}-15^{\circ}$ tilted from the plane of the sky. The primary's $v \sin i \sim$110 km/s combined with this tilt implies that it rotates at a nearly breakup velocity. We detected strong variations of the Balmer and He I emission-line profiles on timescales from days to years. This points to a variable stellar wind of the primary in addition to the presence of a circum-primary gaseous disk. The strength of the absorption-line spectrum along with the optical and near-IR continuum suggest that the primary contributes $\sim$60% of the $V$--band flux, the disk contributes $\sim$30%, and the secondary $\sim$10%. The system parameters, along with the interstellar extinction, suggest a distance of $\sim$1 kpc, that the secondary does not fill its Roche lobe, and that the companions' mass ratio is $q \sim$0.5. Overall, the observed spectral variability and the presence of a strong IR-excess are in agreement with a model of a close binary system that has undergone a non-conservative mass-transfer.

We investigate the dynamical interaction of a central star cluster surrounding a super-massive black hole and a central accretion disk. The dissipative force acting on stars in the disk leads to an enhanced mass flow towards the super-massive black hole and to an asymmetry in the phase space distribution due to the rotating accretion disk. The accretion disk is considered as a stationary Keplerian rotating disk, which is vertically extended in order to employ a fully self-consistent treatment of stellar dynamics including the dissipative force originating from star-gas ram pressure effects. The stellar system is treated with a direct high-accuracy N-body integration code. A star-by-star representation, desirable in N-body simulations, cannot be extended to real particle numbers yet. Hence, we carefully discuss the scaling behavior of our model with regard to particle number and tidal accretion radius. The main idea is to find a family of models for which the ratio of two-body relaxation time and dissipation time (for kinetic energy of stellar orbits) is constant, which then allows us to extrapolate our results to real parameters of galactic nuclei. Our model is derived from basic physical principles and as such it provides insight into the role of physical processes in galactic nuclei, but it should be regarded as a first step towards more realistic and more comprehensive simulations. Nevertheless, the following conclusions appear to be robust: the star accretion rate onto the accretion disk and subsequently onto the super-massive black hole is enhanced by a significant factor compared to purely stellar dynamical systems neglecting the disk. This process leads to enhanced fueling of central disks in active galactic nuclei and to an enhanced rate of tidal stellar disruptions. [Abridged]