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Astron. Astrophys. 347, 932-936 (1999)

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3. Parameters of the orbit

A total of 85 new observations were used to determine new orbital parameters. The radial velocities were measured by cross-correlating the observed spectra with the synthetic spectra calculated with appropriate atmospheric parameters. Such a procedure was successfully applied to e.g. II Peg observations by Berdyugina et al. (1998), where the zero-point velocity of the SOFIN spectrograph of about 0.3 km s-1 for the end of 1996 was estimated. From the SOFIN data we used spectra of seven orders which were free from the Earth atmosphere absorption. No systematic velocity trend among orders was found. The accuracy of the RV measurements averaged from the orders was about 0.15 km s-1, while for the Crimean observations it was about 0.8 km s-1. The measurements are presented in Table 1. Earlier radial velocity measurements of IM Peg, 20 RVs from Harper (1920, 1935) and 19 RVs from Oláh et al. (1998), were used in the period determination. So, the available data presently consist of 125 values spanning from 1919 to 1997.

For our purpose we used the program FOTEL3 developed by Hadrava (1995), which is able to deduce the orbital period value together with other parameters for both circular and eccentric orbits. The first step was to determine new values of the period P and eccentricity e. A first solution was found for all radial velocity measurements with equal weights. The fit allowed different [FORMULA]-velocities for different data sets: [FORMULA](Harper)=-13.14[FORMULA]0.38 km s-1, [FORMULA](Oláh et al.)= -13.31[FORMULA]0.37 km s-1, [FORMULA](Crimea)=-14.88[FORMULA]0.20 km s-1, [FORMULA](SOFIN)=-14.09[FORMULA]0.04 km s-1. The period and eccentricity values were found to be 24[FORMULA]64880[FORMULA]0[FORMULA]00005 and 0.006[FORMULA]0.007, respectively. In comparison to the recently published values of P and e by Oláh et al. (1998), the new value of the period is more accurate, and that of the eccentricity is one order less. Such a small value of the eccentricity can be ignored for the present, and the orbit can be considered as circular.

The accuracy of the data sets is rather different and the new SOFIN measurements are significantly more accurate. For this reason, we assumed the period from the all-data solution and the eccentricity to be zero and found the best solution from the SOFIN data only. The solution is given in Table 2, and the measurements phased with the ephemeris


are shown in Fig. 1. The epoch corresponds to the conjunction with the primary in the back. This ephemeris is used throughout this paper for all observations.


Table 2. Orbital elements

[FIGURE] Fig. 1. Radial velocity measurements phased with the new ephemeris [FORMULA] = 2450342.883 + 24.64880 E. The solution from Table 2 is shown as a solid line. The [FORMULA]-velocities of the sets are reduced to the SOFIN value.

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999