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ABSTRA C T The conjecture is presented that the gap in the distribution of the orbital periods of cataclysmic variables is related to a particular kind of hydromagnetic dynamo, called an interface dynamo, operating near the base of the convective envelope of their secondary components. Such a dynamo is characterized by the spatial separation of the regions where differential rotation and the a effect operate. Unlike conventional dynamos, the linear growth rate of an interface dynamo becomes negative for highly supercritical dynamo numbers, leading to the disappearance of the dynamo action. If such a result, from linear theory, is confirmed by non-linear calculations, it may provide a physical basis for the socalled disrupted magnetic braking hypothesis, invoked to explain the existence of the period gap by several evolutionary models of cataclysmic variables.

This report covers the period July 1, 1996 (the closing date of the reports in the IAU Trans. 23A) through August 31, 1999. Since the reports of Commissions 27 and 42 that follow highlight the research made in the triennium, we limit ourselves here to organizational activities of the Division. The first three sections are devoted to the General Catalogue of Variable Stars (GCVS), the Information Bulletin on Variable Stars (IBVS), and the Archives of Unpublished Observations of Variable Stars, the important projects that serve us all. I am indebted to the authors, listed in the sections headings, for providing these summaries. The problems of naming and classifying variable stars, implicit in Sect. 2, are taken up in Sect. 5. The last section lists meetings sponsored by the Division.

The results of photometric, spectral and radio studies of the flare star EV Lac we obtained during the course of cooperative observations in 1994 and 1995 are presented. A quantitative analysis of the radiation emitted by two powerful flares using the colour-colour diagram confirms the previous conclusion on the essential heterogeneity of matter radiating in optical flares. From simultaneous observations of the star in UBVRI, K and H bands no significant brightness variations in IR were found in coincidence with observed small-amplitude optical flares, except a gradual decrease in the K-band following a 1.0 mag flare in the U-band. A differential spottedness of bright and dark hemispheres of EV Lac is estimated using observations in separate seasons and the total stellar spottedness is determined within the framework of the zonal spottedness model. Spectra of quiet and active states of the star in the blue-green region in 1994 and in the red region in 1995 are described: the characteristics of th...

We present some preliminary results of simultaneous monitoring of the two flare stars, UV Ceti and EQ Peg with EXOSAT and ground based optical spectro-scopy. A strong correlation, found between the Hγ emission and the soft X-ray flux, over several hours of observation of UV Ceti, suggests that much of the low level X-ray flux which has previously been considered ‘quiescent’, in fact derives from small flare events. The implication is that coronal heating in M type dwarfs may result directly from the frequent occurrence of micro-flares.

The present review addresses selected questions on UV Cet-type red-dwarf stars primarily concerning the physics involved in the various aspects and phases of the flare phenomenon, rather than the average activity behaviour of the flare stars. In fact, while flare activity level and general trend are reasonably well established, a fully consistent physical picture of both solar and stellar flare events is still missing. Some recent results are presented with the aim of showing which observations are needed and the relevant role that coordinated multiband studies can play.

We present a statistical analysis of a large sample of homogeneous photoelectric monitoring of the red dwarf EV Lac. The data were collected in 10 years at Catania Astrophysical Observatory on Mt. Etna and consist of 175, 90 and 27 flares detected in the U, B and V bands during a total monitoring times of 861, 947, and 826 hours respectively.

As part of a broad program dedicated to the study of low level stellar activity, in particular microflares and quasi-periodic variations, we have started an exploratory program to detect low level (∼ 0.005 m) variability over time scales of the order of seconds in the optical emission of active stars by the double channel photometer URSULA fed by the 91 cm telescope of the Catania Astrophysical Observatory. Among the several possible targets, we have considered some active flare stars (UV Ceti and BY Dra-type). We describe the first observations and the preliminary results.

The observed kinematics of sunspot groups and a simplified model, which describes the coupling between magnetized and unmagnetized plasmas, are used to derive some information on the rotation of the subphotospheric layers of the Sun. A comparison with present theoretical models and an extrapolation to some aspects of the phenomenology observed in the RS CVn binaries are presented.

We report on the first successful coordinated observations of stellar flares carried out on March 28, 1984 simultaneously over a wide range of wavelengths, from UV to microwaves, using the IUE satellite, three ESO telescopes at La Silla (Chile) and the VLA at Socorro (NM, USA).

SummaryWe present spectral diagnostics for the fluxes of emission lines, in the spectral range 3600-4400 Å, during the cooling phase of stellar flares on dMe stars. Using these diagnostics, electron temperatures have been computed for flares on AD Leonis, Proxima Centauri and UV Ceti. This preliminary model assumes a single flare loop containing a homogeneous, stationary optically-thin flare plasma.

Research on close binary systems has continued at a high level during the past triennium, although the rate of growth is noticeably slower – probably reflecting the cutbacks in funds to which many of us are subject. There have also been changes of emphasis within the field, which are commented on in the pages that follow. These reflect both changing opportunities for observation and the natural development of the subject. In many areas, the time is ripe for a more critical look at ideas that previously seemed adequate.

During the XlXth General Assembly of the IAU in Delhi the number of members of Cotrmission 42 increased to 260. This simply reflects the growing interest and importance of our field. Growing is not only the number of astronomers involved in research on CBS but also the number of papers resulting from that activity. As an example one can quote the numbers of papers listed during the last few years in Sections 117 (Close Binaries), 119 (Eelipsing Binaries), and 120 (Spectroscopic Binaries) of theAstronomy and Astrophysics Abstracts:705(1982), 775(1983), 836(1984), 1080(1985), and 911(1986); note that many additional references could be added to these numbers from other sections. Naturally, such numbers alone do not reflect the quality and even less so the position and significance of the CBS field. Here one could perhaps mention an impressive record of successful research proposals involving requests for the observing time on large, ground based telescopes and on space instruments. Indeed, in spite of a very strong competition from other fields, programs involving CBS are usually placed very high on the priority lists (cf. Sections 2D and 2E). Obviously, the close binary systems, their evolution, and the physical processes which occur in them (accretion, stellar winds, nuclear burning, etc) appear interesting and important not only to those who are involved in their studies but also to astronomers from other fields.

The orthodox models devised to explain the apparent «superluminal expansions» observed in astrophysics—and here briefly summarized and discussed together with the experimental data—do not seem to be too much successful, especially when confronted with the most recent observations, suggesting complicated expansion patterns, even with possible accelerations. At this point it may be, therefore, of some interest to explore the possible alternative models in which actual Superluminal motions take place. To prepare the ground, we start from a variational principle, introduce the elements of a tachyon mechanics within special relativity, and argue about the expected behaviour of tachyonic objects when interacting (gravitationally, for instance) among themselves or with ordinary matter. We then review and develop the simplest «Superluminal models», paying particular attention to theobservations which they would give rise to. We conclude that some of them appear to be physically acceptable and are statistically favoured with respect to the orthodox ones.

A coordinated series of ground-based optical and IUE observations of BY Dra variables was undertaken to follow the spectral variation of these stars over one cycle. In the first series 20 LWR and 19 SWP trailed spectra were taken of AU Mic over a three day period 4–6 August 1980.

The characteristic optical light curves of RS CVn — and BY Dra-type variables are believed to represent non-uniform distribution of dark spots akin to sunspots which are revealed by rotation. By analogy with the Sun, strong magnetic fields probably underlie this phenomenon, extending upwards into the chromosphere and corona, enhancing the emission from regions that overlie the spots. Previous work on the BY Dra variable AU Mic (Ref.1) did not clarify whether the fluxes from chromospheric and transition region lines were rotationally modulated in the sense that the phase of maximum emission was in register with spot visibility or minimum light. This important question prompted the need for further collaborative IUE, optical and radio work.

In this paper the results recently obtained at Catania from the observations of the flare stars PZ Mon, YZ CMi, AD Leo, BD +55°1823, BD +51°2402 and EV Lac are summarized.

The YZ Canis Minoris (Gliese 285), a late-type dwarf star with Balmer emission (dM4.5e), is a member of the UV Ceti class of flare stars. Obtaining good X-ray observations of a dMe star flare is important not only for understanding the physics of flares but also for testing current ideas regarding the similarity between stellar and solar flares. The Einstein X-ray Observatory has made it possible to conduct X-ray observations of dMe stars with unprecedented sensitivity. A description is presented of the results of a program of ground-based optical and radio observations of YZ CMi coordinated with those of the Einstein Observatory. The observations were carried out as part of a coordinated program on October 25, 26, and 27, 1979, when YZ CMi was on the dawn side of the earth. Comprehensive observational data were obtained of an event detected in all three wavelength regions on October 25, 1979.