The telescope in Ondrejov has been equipped with a CCD camera, the other telescopes with photometers with photomultipliers. The newly measured minima are given in Table 1. Their times were determined by the Kwee-van Woerden (1956) method. Other published times of minima have been used in constructing the O-C graphs, and their table can be obtained by e-mail from PM (email@example.com). In all cases where primary and secondary minima were measured or published, the phase of secondary minima is unrecognizable from 0.5, i.e., the systems possess circular orbits.
Note that rather unrealistic values for the orbital period are given in GCVS (2733849) and SAC (2733826).
When older photographic minima are considered, it appears that the period continues to shorten, see Fig. 1. The reason of this shortening is unknown. Since light-time effect could be responsible for it, we took several coudé CCD spectra of this binary with the 2.2 m telescope of the German-Spanish Observatory at Calar Alto, in order to find spectroscopic hints for the existence of a third body. However, no third lines were found, i.e., the luminosity of the eventual third body cannot be larger than about 5% of the integral light of the system in the blue spectral region. From the deviations of the O-C data from linearity the minimum mass of the eventual third body might be roughly estimated as about 1.5 ; the expected luminosity of such a body is of course well under sensitivity of any present spectroscopy.
The scatter of published times of minima is large, see Fig. 2. In this case lines of a third body were found on spectra taken by RL with the coudé auxilliary telescope feeding the 3.6 m telescope coudé spectrograph at ESO La Silla, and with the Calar Alto 2.2 m telescope and its coudé spectrograph. This system will be thoroughly discussed in a forthcoming paper by Lorenz et al. However, the fast changes of O-C values are inexplicable by a light-time effect. In Sect. 3 it is suggested that these changes might be connected with stability of the light curve.
UBV light curves were published by Lorenz et al. (1990). A detailed study of this system by Lorenz et al., including absolute dimensions derived from a recently established radial velocity curve and the light curve analysis, is in preparation.
and phases in the third-body orbit according to
(see Mayer 1987). Two minima given in Table 1 fit the older values of O-C (and the theoretical curve given in Fig. 3 of Mayer 1990) very well, so no changes of periods in the orbit of the eclipsing pair or in the orbit of the third body are apparent.
In the first part of the year 1998, the minima of QZ Car will be observable in South America.
this ephemeris can be used to forecast minimum times in several next years.
a according to Drechsel et al. (1989).
|Figure 4: O-C diagram for AH Cep. The curve corresponds to the light-time effect described by parameters given in Table 2; plus signs are photographic, circles photoelectric data|
AQ Cir: GSC 9015.0071, CPD -642941
comparison: GSC 9015.0147, CPD -642939
Hoffmeister gives 7 times of minimum light and period 057284 (as he remarks, the true period is probably twice as long). From his original data it appears that the accuracy of the period is about 000002 ; this means that at present, the epoch number is not known unambiguously. The large magnitude difference between the variable and comparison given in GSC means that in the time of exposition (JD 2446940.58) the variable had to be close to a minimum. We measured a part of the light curve covering a phase range after a minimum, and one can estimate when the minimum had appeared - perhaps at about HJD 2449012.67.05. Unfortunately, no certain conclusion concerning the period can be reached, and further measurements are necessary. We present our measurements in Fig. 5 in order to facilitate the determination of the period (note that the GSC magnitudes correspond to V magnitudes; we measured B values).
During the time interval covered by observations the period has considerably lengthened (Mayer 1980). However, it appears that the period changed last around E=4700, but then it has been constant, so the presently valid ephemeris can be written as
According to this ephemeris the column (O-C)1 of Table 1 has been calculated and Fig. 6 drawn. The photoelectric times of minima in the interval of epochs from 0 to 4700 fit also a linear ephemeris:
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