[en] Propagation of three-dimensional waves in thin Keplerian polytropic disk is described. The resulting partial differential equation describing linear adiabatic nonaxisymmetric perturbations of the disk is separated into the vertical and radial parts by the use of the WKBJ method. The vertical part forms a Sturm-Liouville-type equation for vertical perturbations. The solutions depend on the radial wavenumbers which are eigenvalues of the problem. The character of resulting waves depends on their location relative to the Lindblad resonances. Beyond the outer resonance there is a finite set of characteristic waves with discrete radial wavelenghts. These waves propagate mainly near the disk surface. Between the resonances solutions of the perturbation equation form an infinite family of waves. They are distributed almost uniformly in the vertical direction. In the adiabatic case the Lindblad resonances are impenetrable walls for the waves which may be reflected and form standing waves. In the general case they may be damped or excited thus producing the disk instability. 13 refs., 7 figs. (author)

[en] We report previously unpublished Einstein X-ray observations of Nova Cygni 1975 (V1500 Cyg), discuss the dsicovery that this system is a polar, and suggested that AM Her-type systems (polars) and DQ Her-type systems (intermediate polars) are the extremes of a continuum of magnetic binaries. Such objects as BG CMi, EX Hya and V1500 Cyg and possibly TV Col, AE Aqr, and CP Pup, whose ''appearance'' depends on individual properties and geometry, are intermediate between the two classes. 48 refs., 1 fig. (author)

[en] A simplified method of calculating classical Cepheid distances is proposed. It is based on photometric data, without the use of the reddenings. By means of results obtained in this way the following problems are discussed: Cepheid double and more numerous aggregates; Properties of the cluster and association Cepheids; The < B-V>₀-logP and ΔV-< B-V>₀ relations in selected parts of the Galaxy. 15 refs., 15 figs., 11 tabs. (author)

[en] CCD photometry on the BV systems is presented for the previously unstudied, rich open cluster Berkeley 39. A color-magnitude diagram for the cluster is derived. The main sequence can be traced down to V = 21.0,4 magnitudes below cluster's turnoff. Through comparison of the cluster CM diagram with the isochrones of Vanden Berg we estimate the following values for cluster age, distance modulus, reddening and metallicity: t = 8 Gyr, (m-M)_v = 13.4, E(B-V) = 0.12, Z = 0.0100. Be 39 is one of the oldest known open clusters. Comparison of two sets of photometry revealed no variable stars candidates. 11 refs., 3 figs., 3 tabs. (author)

[en] It is shown by means of numerical studies that asteroid lightcurves - in particular their amplitudes - are strongly dependent upon the precessional motion of a minor planet. Such a dependence could be used for the determination of the precession of asteroids from the analysis of observed lightcurves. 4 refs., 6 figs., 1 tab. (author)

[en] About 650 blue filter photoelectric observations of 12 and 16 Lacertae, secured in the summer and autumn of 1979 at the Bialkow station of the Wroclaw University Observatory, are presented. An analysis of these data is then carried out. In the case of 12 Lacertae no substantial change in the frequency spectrum is detected relative to the earlier results. As far as 16 Lacertae is concerned, it is found that the decrease of all three pulsation amplitudes, noted in 1977, has probably continued through 1979. The observations of 16 Lacertae include fragments of three successive eclipses of the star by its invisible companion. These new data confirm the ephemeris derived previously from the 1965 and 1977 observations. 8 refs., 5 figs., 8 tabs. (author)

[en] The optical variability of 14 Quasi Stellar Objects has been examined on plates taken with the Asiago Observatory telescopes. Five of them show large variations while 9 do not appear to vary. 9 refs., 6 figs., 2 tabs. (author)

[en] Basing on the solution of one-body Schwarzschild problem, the relativistic terms were included to the equations of motion of a minor planet or comet. It appeared that the using of Painleve's coordinates allowed to write the equations of motion in a very simple form. Equations of motion as well as the commonly used equations based on the Schwarzschild isotropic and nonisotropic line elements were numerically integrated by the recurrent power series method. The results of integration of the motion of Mercury and of the minor planet Icarus show strictly the perihelion motion predicted by the general relativity theory. The relativistic effects in the motion of some minor planets and comets were examined too. (author)

[en] Chromospheric and transition-region activity of the intrinsically X-ray brightest contact binary XY Leo were studied using the low resolution IUE spectra: one long exposure SWP spectrum and four LWR spectra, the latter obtained at both minima and at one of the orbital quadratures. Both components seem to be similarly covered by active regions as judged from the constancy of the relative flux f(Mg2)/f(bol) = (17.6±0.6)x10^-5 (orbital average). The mean line fluxes of a few chromospheric and transition-region lines visible in the SWP spectrum were found at levels similar to those in other contact binaries of very short orbital periods. 17 refs., 2 figs., 3 tabs. (author)

[en] A linear adiabatic and non-adiabatic survey of pulsation periods (up to the fourth overtone) was performed in the parameter ranges relevant for double-mode RR Lyrae stars. It was shown that among the lowest order resonances between the frequencies of the normal modes the closest is the 2:1 resonance between the fundamental and third overtone. It is argued that proper inclusion of non-adiabaticity may further improve the situation for this resonance. However, there is a problem with satisfying simultaneously constrains imposed by observed periods and resonance condition. As a by-product of our survey it was shown, that systematic differences exist between the different pulsational codes, causing uncertainty of 10-20 percent in mass determination. 22 refs., 8 figs., 1 tab. (author)