An extensive search for ET-systems was undertaken in the literature.
Only systems for which solutions of the light curve giving the semi-detached
configuration (or very close to it) and preferably also the spectroscopic
observations exist were used for the following analysis. The number of the
suitable systems was also inevitably lowered by the requirement of a good
coverage by the timings amounting several decades. The parameters of the
final ensemble of binaries are summarized in Table 1 where
also the sources are cited. The radii of the gainers in Lyr and
RY Sct, supposed to be veiled by the accretion disk, were obtained by
an interpolation in the table given by
Harmanec (1988). These radii were
determined for the masses and inferred spectral types given in the cited
papers.
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Errors of the photoel. timings are usually smaller than the symbols
used and therefore do not represent serious problem. Unfortunately, the
accuracies of the visual and photographic timings, generally having larger
scatter, are not available in many cases. One possible way how to assess them
is a visual inspection of the O-C diagrams. As can be seen from the attached
figures most these data are in a good agreement with the photoelectric ones.
Accuracy of the visual and photographic timings was further evaluated by
linear fits of these data inside the well covered segments of the O-C curves
where the period P could be approximated as roughly constant. The 1errors of these timings were then determined from the residuals of the fit
using the least squares method.
In general, the course of the O-C changes is often complicated and it is not possible to give any unique method for their analysis. An overview and discussion of the existing methods and general considerations of analysis of an O-C diagram can be found in Simon (1997c). The visibility of the period change becomes considerably suppressed with the growing slope of the O-C curve on the plot. The period lengths were therefore calculated to give the slope close to zero in a large part of the curve. The photoelectric observations which well define the O-C curve are usually found in the second half of the interval covered by the timings in the systems analysed here. In this case it often appears advantageous to keep the slope of these photoelectric data close to zero since the eventual period change (namely if it is inferred from the old timings obtained by other methods) can be more easily resolved and evaluated.
We admit that indication of the period change in a given binary is
dependent on the chosen criteria to some extent. We will consider the period
change in a given binary as present if it can be resolved by the visual
inspection of the O-C diagram which is constructed according to the
principles listed above. Its course must be defined by multiple
observations. Polynomial fits to the O-C values (e.g.
Kalimeris et al. 1994)
appear to be a plausible method of analysis of the period change in our case.
We note in advance that such fits to the O-C values of the respective
systems analysed here have shown that even the most variable periods could be
plausibly matched by the second order polynomials. Comparison of the sum
of the squares of the residuals of the linear () and parabolic
(
) fits of the data can then serve as another clue in
assessment of the period change. We will therefore introduce the ratio
; the larger S, the better defined variation. The results,
discussed for the respective systems below, have shown that monotonic
change of the O-C values can be usually well recognized for S =1.2; this
figure may be accepted as a typical limiting value above which the period can
be regarded as variable. Discussions of the data and variability of the
periods of the respective systems are given below.
The O-C values in the diagrams plotted here are expressed in days as is usual. Nevertheless, it should be noted that the range of the period lengths in the respective systems analysed here is high and amounts about one to ten. The longer periods generally imply longer duration of the eclipse what leads to a larger scatter of the O-C values. This scatter is more pronounced especially for the timings obtained by other methods than photoelectric. If the variations of the O-C values are expressed as a fraction of period P instead of days, eventually if the O-C values are scaled down according to the respective period lengths of the systems then it can be shown that these values for the different systems usually display comparable scatter.
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