The star forming region NGC 7129 was first studied by Herbig (1960), who discovered two bright (BD+65° 1637 and LkHα234) and some weaker Hα emission stars. Subsequent investigations in this region led to the detection of five Herbig-Haro objects (Strom et al. 1974, Gyulbudaghian et al. 1978, Eiroa et al. 1992) and one T Tauri star, V350 Cep (Gyulbudaghian & Sarkissian 1977). Some non-stable stars in NGC 7129 are surrounded by reflection and cometary nebulae.

Our Hα-emission survey (Semkov &: Tsvetkov 1986) revealed an interesting irregular variable star in the dark clouds near the emission nebulae. On objective prism plates the star shows very strong Hα-emission and was included in our list of such stars as No. 7. An identification chart and the coordinates can be found in Semkov (1993a). The observations reported here include long-time photographic photometry and CCD spectroscopic and photometric observations.

Quasi-simultaneous optical and IR photometric (BVRIJHK) and optical polarimetric (BVRI) observations were carried out at the 1 m telescope of Assy Observatory (Institute of Astrophysics of the Kazakhstan Academy of Sciences). The photometer-polarimeter of the Pulkovo Observatory (Bergner et al. 1988) was used for the study. All our measurements were done with apertures of 26" and 52".

HDE 259431. Our data show a strong polarimetric variability Δp = 2% – 3% and ΔΘ 30° (especially in UV and optical region). Although the general nature of polarization is connected with the dust disk around the star (Dzhakusheva et al. 1988), the polarimetric variability probably has an another origin. This is due to the fact that we have observed a strong polarimetric variability on a time-scale of up to several hours (Dzhakusheva et al. 1988). Incidentally, this rapid polarimetric variability has no correspondence in photometry.

We investigated the long-term photometrical behaviour of the EXors V1118 Ori and V1143 Ori on the basis of the Sonneberg Plate Archive. About 2800 plates were checked for outbursts of these stars. We found several dozens of plates where the stars appeared to be distinctly brighter than the plate limit. To get magnitudes we have scanned those plates and applied a special method of brightness determination which is currently used at Sonneberg Observatory. Because of the small number of plates we were able to extract only crude estimates of some typical time scales of outburst and quiescent phases.

Short-term fadings (dips) in the range of a few seconds with amplitudes from 0m.3 to 1m.2 were discovered in the two T Orions type variables DD Ser and AB Aur. Similar features were observed in T Tau and R CrB by other observers.

Light curve analysis for the decay phase of novae gives estimates of the white dwaif mass, the luminosity (or distance) and the chemical composition of ejecta. Differences in these parameters among nova subclass, i.e. fast, slow and recurrent, are briefly summarized to speculate on their cause. Helium shell flashes occur in mass-increasing white dwarfs, their theoretical light curves are given in UV and visual wavelength bands. An evolutional scenario for fast, slow and recurrent, and slow novae through a helium nova/EUV source-stage toward a type Ia supernova/neutron star formation-stage is given.

In autumn 1979, the young W. Wenzel - at this time he was only 50 years old - came to the Byurakan meeting on flare stars and informed the participants about a recently discovered flaring object: the Kuwano-Honda object = PU Vul. During that time, FU Ori and related stars were the most intriguing variable objects, and PU Vul had shown some similar features such as the rate of brightening and the spectrum at maximum. It turned out to be an extremely interesting object, and the Crimean team has studied it for the last 15 years. Over this time, this team included T.S. Belyakina, N.I. Bondar, K.K. Chuvaev, Yu.S. Efimov, R.E. Gershberg, V.I. Krasnobabtsev, E.P. Pavlenko, P.P. Petrov, I.S. Savanov, N.I. Shakhovskaya, N.M. Shakhovskoy, A.G. Shcherbakov and V.A. Shcherbakov, and we collaborated with V.I. Shenavrin from Russia, with D. Chochol, J. Grygar and L. Hric from former Czechoslovakia and with V. Piirola, I. Tuominen, M. Poutanen and J. Huovelin from Finland.

Now the number of publications on PU Vul approaches one hundred. It was established that PU Vul is a symbiotic nova, the object among the known variables of this type, whose explosion became observable most recently (with the possible exception of Wakuda’s object in Sgr, Grebel et al. 1994), and many details have been observed from the very beginning of the flare. I intend to list the most interesting findings and the most intriguing remaining mysteries of PU Vul.

We present preliminary results of observations of the Hα, HeI 5876 and NaI D lines profiles in spectra of PU Vul. The variability of profiles is confronted with the photometric behaviour. The observations were carried out with a CCD camera, installed in the coudé spectrograph of the 2.6 m Shajn telescope. 68 spectra of the Hα line and 25 spectra in the region of the HeI and NaI lines were obtained.

Fig. 1 (left panel) presents the evolution of the Hα line from 1985 to 1994, displaying selected typical profiles in different years. The profiles are shown in a heliocentric wavelength scale and in logarithms of intensity. Variations of line intensity, line width, as well as radial velocity of the centre of weight of the total emission and absorption components are observed.

Stronger and more complex variability is observed in the region of the HeI and NaI D lines. The strong P Cyg type absorption in the NaI D lines during 1985-1987 disappeared in later spectra as a result of total ionization of NaI (Fig. 1., right panel). Contrary to that behaviour, HeI 5876 shows variations from a pure absorption profile to a very strong and wide emission profile.

CH Cygni has been observed since about one decade at the Bulgarian National Astronomical Observatory Rozhen. UBV photometry was obtained with the 60 cm telescope. The computer controlled photometer is equipped with an EMI 9789 QB multiplier, a set of u,b,v filters and a diaphragm of 28". The reduction includes corrections for dead-time and background, differential extinction and transformation to the standard UBV system.

Observations between 1991 and 1994 are displayed in Fig. 1 and show the new outburst, which is still going on. The previous major outburst lasted until 1984, and CH Cyg declined slowly between 1985 and 1988. In 1988, it reached its minimum in brightness and activity. The cause for the activity is probably mass transfer from the M giant component onto the hot companion. In this model, only the M star is seen during activity minimum. In July 1988, the mean brightness was V = 8.50, B - V = 1.39 and U - B = 1.48. Since 1989, some erratic activities appeared, including a hot variable continuum, broad and variable emissions in the Balmer lines and rapid light variations (Leedjarv 1990, Tomov and Mikolajewski 1992, Kuczawska et al. 1992, Panov and Ivanova 1992). Since 1991, a trend of increasing activity is observed, which persisted also in 1994. The brightening of CH Cyg of about 5m in U in 1994 with respect to 1991 shows that it undergoes another major outburst. Moreover, it exhibits light variations with different other time scales, minutes (flickering), days and weeks, probably resulting from the mass-transfer. Fig. 2 showns an example of a short time variation on October 1, 1994. The flickering of CH Cyg was very pronounced in the U, B and V filters and the amplitudes reached several 0m.1 in about half an hour. More observations of this interesting system are planned.

Photoelectric UBV-observations of the well known symbiotic star AG Draconis were obtained between 1982 and 1984 with the 60cm photoelectric reflector II of Sonneberg Observatory. The light curve shows two large optical outbursts: (at JD = 244 5000…5500 and JD = 9000…9500) and two smaller eruptions (at JD = 6200, 6600). The recent outburst is the largest one since 1952.

During the quiet stage a significant variability with an amplitude of about one magnitude is only visible in the U-band. L. Meinunger (1979) detected a period of 554 days and calculated the following ephemeris:

Umax = 243 8900 + 554 · E

All outbursts and smaller eruptions are correlated with the period of the U-maxima, the outburst maxima always coincide with U-maxima of the quiet stage, calculated by the above formula.

Space densities of novae, dwarf novae and novalike stars are derived on the basis of the most recent catalogue of cataclysmic variables and improved values of absolute magnitudes. Assuming that these objects represent different stages of the same type of variable, changing over short-term cycles, we derive an average time interval of 10000 years between two subsequent nova eruptions. During this time, the cataclysmic variable remains almost 2000 years in the novalike stage and the remaining time in the dwarf nova stage, in which a total of nearly 3 · 10−5M⊙ is accreted on the white dwarf primary. This amount is comparable to the average mass of ejected nova shells.

AE Aquarii is a magnetic cataclysmic variable containing a white dwarf and a K3-K7 star which lies slightly above the main sequence. The white dwarf is the most rapidly rotating known (Prot ≃ 33.08 s, Patterson 1979), and it is the most strongly asynchronous with its revolution (Porb = 9.88 hr). The white dwarf accretes matter from the K star, which approximately fills the Roche lobe. AE Aqr exhibits flares in the soft X-rays, the ultra-violet, and almost continuously in the visible and the radio regimes. Rapid optical and TeV γ-ray bursts have also been discovered, which are modulated with the period of the white dwarf and at half of this period (de Jager & Meintjes 1993). This modulation, also found in X-rays, is interpreted as the accretion of matter onto the white dwarf’s magnetic poles. The strength of the white dwarf’s magnetic field is not well-determined, it is estimated to be ∼ 6.104 - 105 G (Lamb & Patterson 1983, Cropper 1986) at the white dwarf’s surface. Eracleous et al. (1994) recently suggested that the magnetic dipole axis lies close to the equatorial plane (∼ 20°). De Jager et al. (1994) discovered a rapid spin down of the white dwarf leading to a spin down power which exceeds the accretion power. They suggest that a significant fraction of the spin down power may be converted to the acceleration of particles, which may explain the radio and the γ-ray emissions. Both the characteristics of the optical flares and the existence of TeV γ-rays suggest a relation with the non-thermal radio flares.

AE Aqr is a close non-eclipsing binary system with an orbital period Porb ≈ 9.88 hr, eccentricity of the orbit e = 0.02 (Chincarini & Walker 1981) and a mass ratio q = 1.14 (Reinsch & Beuermann 1994), situated at the distance of ∼ 84 pc (Van Paradijs et al. 1989). The secondary is K3 red dwarf on or close to the main sequence (Bruch 1991). The primary cannot be observed directly. However, the stable photometric oscillations at a period of 33 s (and at half this period) in the optical (Patterson 1979), UV (Eracleous et al. 1994), X-rays (Patterson et al. 1980) and, probably, VHE γ-rays (Meintjes et al. 1992, Bowden et al. 1992) leave no doubt that the primary is a rapidly rotating magnetized compact object. The observed rate of deceleration of the primary rotation is Ṗ = 5.64 × 10−14 ss−1 (De Jager et al. 1994), and its mass is m1sin3i (0.56 ± 0.03) M⊙ (Reinsch & Beuermann 1994). The lack of eclipses allows to put a lower limit to the mass of the primary m1 ≥ 0.62M⊙.

AE Aqr emits detectable radiation in all parts of the electromagnetic spectrum. In radio and γ-rays it is a powerful non-thermal source. However, in the optical, UV and X-rays the radiation is likely to be thermal and can be well explained by gas accretion onto a compact star (Patterson 1979, Patterson et al. 1980). The observed impulse profile of the pulsating component (Eracleous et al. 1994) looks similar to that of the accreting X-ray pulsars.

The most striking feature of the close binary system AE Aqr is its unique photometric behaviour - irregular alternations of quiet and active phases without noticeable transition periods. Although extensive photometric observations of AE Aqr have been carried out for more than forty years (see Bruch 1991 and references therein), the mechanism of its flaring activity is still poorly understood and the star - although generally classified as a cataclysmic variable - does not fit well into any of the subclasses.

According to us, much insight into this peculiar object can be gained by an investigation of the connections between the flaring activity of AE Aqr and the strictly periodic oscillations at 33s, identified as the spin period of the primary. The latter have been seen in optical (Patterson 1979) and UV light (Eracleous et al. 1994), in X-rays (Patterson et al. 1980) and in TeV γ rays (Meintjes et al. 1992). There are contradictory reports about the relation between their amplitude and the flaring activity in AE Aqr: Patterson (1979) noted that the amplitude of the pulsations increases during active phases, while Eracleous et al. (1994) observed that their characteristics are not strongly affected by the flares.

Detected from the radio to the γ-ray bands, AE Aquarii is an extraordinary cataclysmic variable. It is a relatively bright (V ∼11.4), non-eclipsing binary with an orbital period of 9h.9. The mass-donor secondary star is of approximate spectral type K5V and contributes about ∼ 80% of the total flux in the optical. While the mass ratio is well determined at q = M2/M1 = 0.646, the individual stellar masses are poorly known since the inclination is unknown. AE Aqr’s flickering behavior is unique among CVs, alternating from periods of very violent activity to near total quiescence. Large amplitude flares have been detected in the radio, optical and UV. Previous work has shown the optical flare spectrum to be similar to stellar flare spectra.

Four nights of exactly simultaneous Hα spectroscopy and photometry were obtained on the 2.5 and 1.5m telescopes at Mt. Wilson on 27-30 July 1982. The simultaneity allowed us to renomalize the spectroscopy to agree with the photometry, giving us spectrophotometric data (spectral resolution ∼ 50 km s−, time resolution ∼ 68). A total of ∼ 17h of data were obtained, resulting in 10,098 spectra.

In this work, we study orbital variations in the ultraviolet resonance lines of the SU UMa type dwarf nova VW Hyi examining spectra in the data archive of the International Ultraviolet Explorer (IUE). We show that VW Hyi exhibits an orbital and long term variability in terms of UV line profiles and continuum flux at 1450 Å during superoutburst.

Dwarf novae and nova-like stars are a sub-group of the class of cataclysmic variables. Most of these stars show high and low brightness states (maxima or outbursts and minima or quiescent states, respectively). They all are short-period interacting binaries consisting of a white dwarf primary, a late-type Roche-lobe filling secondary, and an accretion disk around the white dwarf. For details on observations of cataclysmic variables and their theoretical explanations and modelling, the interested reader is referred to recent reviews.

The main problem when trying to understand these systems is the wealth of patterns presented by the whole group: no two objects are nearly identical and it is hard to say which the ‘typical’ features are. As a result of this, from an individual observation, or from observations of a few systems only, it is not possible to decide which of the observed features are characteristic of the entire class, and which are specific of individual members. The only way is to investigate many different members of the same class, arrange them according to their different physical parameters, and try to determine what the general characteristics are.

Many observations indicate that fast rotating late type stars show magnetic activity. We therefore argue that some of the secondary stars in cataclysmic binaries might also have magnetic fields. Such magnetic fields would reach over the accretion disk around the white dwarf primary. We investigate their effect on dwarf nova outbursts. The magnetic field lines will penetrate the disk and remove angular momentum. This shifts the accumulation of mass towards the inner disk, closer to the white dwarf, and therefore leads to a different outburst behaviour, which can be recognized in observed light curves of dwarf novae. If a magnetic field of the order 50 - 100 gauss is acting on the accretion disk, we expect narrower and more frequent outbursts as compared to the non-magnetic case. Outburst records of three dwarf novae above the period gap (P > 3h), whose long-term light curves are well covered, were used to search for traces of magnetic activity. All three cases display a pronounced bimodality in the distribution of their outburst width, i.e. either narrow or wide outbursts occur. We found evidence for a cyclic behaviour in one case, SS Aur: possible “magnetic episodes” repeating every 18 years reveal epochs with abnormally frequent narrow outbursts and nearly or totally missing wide ones. There are also indications for a similar behaviour of SS Cyg with a 7-years cycle, but with less pronounced periodicity. The third case, U Gem, does not show clear evidence of magnetic activity although we found some indications for a transitory oscillation of the width of wide outbursts after 1926, with a period of 13.6 years. The behaviour of SS Aur and SS Cyg resembles the theoretical predictions, there is, however, an important difference: Throughout the entire cycle, marked by the “magnetic episodes”, neither wide nor narrow outbursts alter their mean light curves: magnetic fields seem to affect only the observed proportion in the frequency of both outburst types without altering the light curves of individual outbursts. Remarkably, most of the “anomalous outbursts” (which are characterized by an abnormally slow rising branch to an outburst) occur also at or near the “magnetic episodes”.

This report provides a summary of flares observed visually in the dwarf novae V442 Cen, OY Car, TU Men and Z Cha. The flares precede an outburst by an average of around ten days.

Stochastic variations on short time scales called flickering are a characteristic feature in the light curves of CVs. Their amplitudes indicate that their origin must be related to one of the principle light sources of these systems. But the physical mechanism behind the flickering is unknown. New attempts to quantify the properties of flickering are presented, and a consistent scenario for its origin is outlined.

Cataclysmic variables of the AM Her type are close binary systems with accretion onto a highly magnetic (a few times 107 G) white dwarf. In these systems the magnetic field is strong enough to prevent the formation of an accretion disc. The accretion flow is instead funneled directly onto the polar caps where the plasma is shocked and radiates intense optical cyclotron radiation as well as bremsstrahlung at X-ray energies. The cyclotron radiation component, which dominates in the optical, is highly polarized (5-35%). The cyclotron emission exhibits intensity variations on a large range of time scales. This include quasi-periodic oscillations, flickering and long term changes between brightness states. The observed source intensity is also modulated by the white dwarf rotation which is synchronized with the orbital period (101 min for V834 Cen). The flickering is believed to be due to inhomogeneities in the accretion flow. These could either be density variations in a continous flow or it could be in the form of independent blobs accreting over the polar cap region. As blobs hit the white dwarf surface and radiate their accretion energy they should be seen as shot noise. For a theoretical discussion of the fate of such blobs see Frank et al. (1988). A modeling of the optical variability in AM Herculis, in terms of a shot noise process was made already by Panek (1980). It was found that a consistent model could be constructed by randomly occurring 708 – 908 rectangular shots, with on the average a few overlapping shots. The power spectrum had a v−2 shape above 0.02 Hz as expected, although it is not clear whether a break in the slope of the power spectrum was actually seen around 0.01 Hz. The shot overlap meant that individual shots could not be resolved directly in the light curve. This limits the precision and uniqueness by which shot parameters can be determined. However, in our observational data of AM Her objects we have found occasions when the character of the flickering has changed to become much more “flarelike”, suggesting that individual shots might be seen in these cases. This change is also associated with a change in the shape of the power spectrum, in particular a higher power level and a steepening at the high frequency end. Most importantly, there is a distinct break in the power spectra around 0.03 Hz, indicative of a characteristic time scale in the flickering noise. In addition to a power spectrum analysis we have also made a direct modeling of individual flares in the light curve. The distribution and correlations of the fitted pulse parameters were studied and compared with simulated data. The primary aim was to investigate whether the flares are really individual shots or produced by a random superposition of more frequent smaller amplitude shots.