Peter Rovithis

Starspots and photometric noise on observed minus calculated (O{C) diagrams

The eects of starspots on the O{C diagrams of eclipsing binaries are investigated, with a special em- phasis on contact binaries. It is shown that starspots modulate the O{C dierences in a fundamentally dierent way than real orbital period variations. Migrating starspots can only introduce high frequency, low amplitude disturbances (generally less than 0:01 d), whose characteristics are numerically specied for typical contact bina- ries. It becomes evident that the observed systematic short timescale variations of the O{C diagrams of active contact binaries (even those like VW Cep) cannot be attributed to starspots. Furthermore, the basic property of non-linear modulation of O{C dierences is emphasised. This non-linearity has serious consequences for the widespread traditional method of O{C diagram analysis. This traditional method is found inappropriate even on a fundamental level, and its mathematical and physical inconsistencies are reviewed.

Evolutionary state of W UMa-type systems

The evolutionary status of a sample of 31 W UMa-type systems (20 of W, 9 of A, and 2 of A/W sub-class) is examined and discussed on the basis of their positions in the H-R, mass-radius and mass-luminosity diagrams. The absolute elements of these systems have been evaluated by use of their spectroscopic and photometric data as they were found in the literature. Since different analysis methods have been used by the various investigators this gives the opportunity to make different H-R, mass-radius and mass-luminosity diagrams, according to the method used, and compare the results.Moreover, from the spectral type-period diagram of those systems that have spectral types from O to G2 it was found that all of them lie in the evolved region. Finally, the log (Ll/Ls) log (Ml/Ms) diagrams are presented and discussed.

Ubvr light curves of 44i Bootis during 1978

In this paper theU, B, V, R light curves of the eclipsing variable 44i Boo, obtained during 1978, are represented and discussed.

A NEW LARGE SUPERNOVA REMNANT IN THE SOUTHERN SKY

A faint filamentary structure on a Southern Schmidt plate is thought to be a supernova remnant.

The solar differential rotation from the eightteenth solar cycle

The sidereal daily rotation of the Sun, ζ(φ), depends on the data used. From an appropriate selection of the data — sunspots with regular motion — it is found that ζ(φ)=14.31–2.70 sin2 φ, where φ denotes the heliographic latitude. Moreover, it seems that there is a variation, of the order of 3%, with the solar activity.

The Eclipsing Binary BX Andromedae and its Orbital Period Behaviour

The orbital period variations of the eclipsing binary BX And are examined analysing its (O–C) diagram 1) with the standard method, in which the minima times are fitted by the quadratic ephemeris combined with an assumed light-time effect, and 2) with the first continuous method. The results from the use of the two methods are, as was expected, different.

Observations and Light Curve Analysis of Eclipsing Binaries of W UMa — Type

New light curves in two wavelengths (B and V) of eleven W UMa- type systems, obtained at Kryonerion Observatory in Greece, are presented. The light curves of these systems are analysed by using Frequency Domain techniques and new geometric and photometric elements are given. These elements are combined with the available spectroscopic data of the systems and new absolute elements are derived. Using the latter elements the evolutionary status of these systems is considered on the basis of mass-radius and mass-luminosity diagrams, and the results are compared with previous studies, where the physical parameters of these systems are based on photometric elements derived by other methods.

The eclipsing binary BW Draconis

In this paper theB- andV-light curves of the eclipsing binary BW Draconis made at Kryonerion Station of the National Observatory of Athens during 1980 and 1981 are presented.