Results of the study of the color indices and luminosities of 17 optical afterglows (OAs) of GRBs are presented. We show that the color variations during the decline of OAs (except for GRB000131) are relatively small during t − T0 < 10 days and allow a comparison among them, even for the less densely sampled OAs. The colors in the observer frame, corrected for the Galactic reddening, concentrate at (V − R)0 = 0.40 ± 0.13, (R − I)0 = 0.46 ± 0.18, (B − V )0 = 0.47 ± 0.17. The color evolution of the OAs is negligible although their brightness declines by several magnitudes during the considered time interval. Such a strong concentration of the color indices also suggests that the intrinsic reddening (inside their host galaxies) must be quite similar and relatively small for all these events. The absolute brightness of OAs in the observer frame, corrected for the host galaxy, lies within MR 0 = −26.5 to −22.2 for (t − T0)rest = 0.25 days. This spread of MR 0 is not significantly influenced by the shifts of λ, caused by the different redshift z of the respective OAs. The general decline rate of the OA sample considered here seems to be independent of the absolute optical brightness of the OA, measured at some t − T0 identical for all OAs, and the light curves of all events are almost parallel, when corrected for the redshift-induced time dilation.

We present photometric and spectroscopic observations of SN 2000ch, an unusual and extremely luminous variable star located in the galaxy NGC 3432. The object was discovered on 2000 May 3.2 during the course of the Lick Observatory Supernova Search, at an unfiltered magnitude of about 17.4. Pre-discovery images obtained in 1997, 1998, and 2000 April show the object with R = 19.2 − 19.5 mag. Optical spectra obtained beginning on 2000 May 6 show a smooth, flat continuum and strong, broad hydrogen Balmer emission lines at wavelengths consistent with the catalogued redshift of NGC 3432, strengthening the association of the variable with the galaxy. Photometric monitoring reveals a complex and erratic light curve over a time span of ∼10 days. Subsequent optical spectra over the next ∼ 3 months continued to show strong Balmer emission lines with a mean full-width at half-maximum intensity ∼ 1550 km s −1 and a distinct red asymmetry. A spectrum obtained 9 months after the outburst is similar to the previous spectra, but the integrated flux in Hα is nearly half that observed during the outburst. The object's photometric behavior, spectrum, and luminosity suggest that it is a very massive and luminous variable star and might be related to some luminous blue variable stars such as η Carinae and SN 1997bs in NGC 3627. The brightest apparent magnitude implies an absolute magnitude of M V ≈ −12.7 at the distance of NGC 3432, a value which is comparable to η Carinae during its outburst in the mid-nineteenth century.

Broad-band optical observations of the extraordinarily bright optical afterglow of the intense gamma-ray burst GRB 991208 started ∼ 2.1 days after the event and continued until 4 Apr 2000. The flux decay constant of the optical afterglow in the R-band is −2.30 ± 0.07 up to ∼ 5 days, which Send offprint requests to: A. J. Castro-Tirado (ajct@laeff.esa.es)is very likely due to the jet effect, and after that it is followed by a much steeper decay with constant −3.2 ± 0.2, the fastest one ever seen in a GRB optical afterglow. A negative detection in several all-sky films taken simultaneously to the event, that otherwise would have reached naked eye brightness, implies either a previous additional break prior to ∼ 2 days after the 2 A. J. Castro-Tirado et al.:The extraordinarily bright optical afterglow of GRB 991208.

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Abstract. This analysis has shown that the intermediate polar GK Per experienced a very striking evolution. Its outbursts became wider and brighter in the last five decades. These changes were accompanied by striking variations of the recurrence time TC, from 385 days within the years 1948-1967 to 890 days in 1970. Nowadays, TC displays a strong trend of linear increase. Decrease of irradiation of the disk by the WD, combined with the decrease of viscosity, offers a plausible explanation. It is argued that variations of the mass transfer rate are unlikely to play a major role. The morphology of the outburst light curves in the optical and the X-ray region is also studied. The decay branch in the optical remains remarkably similar for all the events while the largest changes of the light curve occur in the rising branch. This can be explained if the thermal instability may start at different distances from the disk center. The quiescent level of brightness does not display any secular trend in recent decades but a wave on the time scale of about 30 years with the full amplitude of 0.3 mv, probably due to activity of the cool star, is detected.

Abstract. Analysis of the AAVSO and AFOEV visual data (more than 6000 one-day means) revealed that V Sge displayed intervals of the suppressed brightness variations (flat segments), interchanging with intervals of the pronounced changes (active segments). The borderlines of the segments are sharp and are just several days long while the length of the segment is several years. The character of the brightness variations in the active segments evolves and depends on the mean level of brightness in the given segment. The low level gives rise to the separated relatively narrow outbursts while the high state/low state transitions occur in segments with higher mean brightness. We discuss the long-term activity in terms of the divergent models for V Sge and conclude that the mass accreting white-dwarf primary and variable mass transfer offer a viable explanation for the photometric activity of V Sge. Our analysis therefore brings an additional support to the model by Williams et al. (1986) and Patterson et al. (1998).

Abstract. The analysis of the recurrence time TC of outbursts in Aql X-1, using the method of O-C residuals, revealed that the character of the O-C curves bears a striking similarity to that of dwarf novae. It means that variations of TC are large, but generally not chaotic and long-term trends in the O-C curves can be resolved. The prevailing value of TC is 211 days but the evolution of TC shows several large jumps. The mean TC varies by more than 1:2 and the switches occur within just a single epoch. A comprehensive study of the morphology of the light curves from ASM/RXT E revealed that although the respective outbursts in Aql X-1 differ in both their duration and the maximum intensity, their rising and decay branches display remarkable similarities. The rising branches are exponential. The slope of the decay is broken in some events, with the final phase having a steeper slope and being roughly linear. We argue that just the inner disk is irradiated during outburst and that viscous heating plays a big role. The evolution of the hardness ratios HR1 and HR2 displays common characteristics for the respective outbursts observed by ASM/RXT E. HR1 at the peak intensity of the more intense outbursts tends to be harder. Similar behaviour is also apparent for HR2. An asymmetric course (hysteresis) of HR2 with respect to the moment of maximum intensity is apparent for some outbursts, the largest value of HR2 occurring on the rising branch. The observed behaviour is discussed in terms of the thermal instability model and variations of the thermal and power-law spectral components.

DG CVn is a binary system in which one of the components is an M type dwarf ultra fast rotator, only three of which are known in the solar neighborhood. Observations of DG CVn by the Swift satellite and several ground-based observatories during its super-flare event on 2 014 allowed us to perform a complete hard X-ray -optical follow-up of a super-flare from the red-dwarf star. The observations support the fact that the super-flare can be explained by the presence of (a) large active region(s) on the surface of the star. Such activity is similar to the most extreme solar flaring events. This points towards a plausible extrapolation between the behaviour from the most active red-dwarf stars and the processes occurring in the Sun.