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2 Observation

Photoelectric photometry in B and V bands for YY Eri was conducted over four nights in November, 1984, with the 35-cm Cassegrain reflector of the Yunnan Observatory in China. A 1p21 photomultiplier was used in the observations. The two filters employed were the following: a Corning 5030 of stock thickness cemented with 2 mm Schott GG-13 for the blue region, and a Corning 3389 of Stock thickness for the yellow region. A circular diaphragm of 30'' was used thoughout, and an integration time of 20 seconds for both stars and sky was chosen.

BD $-11^{\circ}831$ and BD $-12^{\circ}828$ were chosen as comparison star and check star, respectively, and measured alternatively between the observations of the binary. Nightly extinction coefficients were determined from observations of the comparison star, and the differential extinction correction were made. The observations were transformed into the UBV system with the aid of six standard stars in the Pleiades during the period of observations. The magnitude differences between the check star and the comparison star were constant within probable errors of $\pm{0.010}$ mag(V) and $\pm{0.012}$ mag(B) during the observations.

From the observations, three times of primary minimum light and two times of secondary minimum light were derived by means of polynomial fitting and are listed in Table 1. Because of the complexities in the orbital period change of YY Eri, to combine the observations, we have adopted the light ephemeris found by least square method from the minima in the Table 1 and the minima published by Marco et al. (1993) as follows:

{\rm Min.I=JD(hel.)}2446028.1953+0.3216498\times{E}.\end{displaymath}

In each colour 440 individual observations were secured. The magnitude differences in the sense of YY Eri minus BD $-11^{\circ}831$together with their heliocentric Julian dates are listed in Table 2 for V and Table 3 for B. The light curves of YY Eri are shown in Fig. 1 in solid points. All phase is covered at least twice. The light curves appear to exhibit a typical O'Connell effect, with Maximum I brighter than Maximum II by 0.042 mag(V) and 0.048 mag(B) and Maximum I shifting to phase of 0.27.

\includegraphics [width=7.8cm]{ds1450f1.eps}\end{figure} Figure 1: Observed (points) and computed BV light curves (solid lines) of YY Eri, assuming a standard photometric model. To enhance clarity, the two light curves have been displaced vertically by arbitrary amounts

Table 1: Minima of YY Eri

 JD.Hel. & Min.-type & $E$\...
 ...I & 0 & $-$0.0008 \\  8.3558(7) & II & 0.5 & $-$0.0003 \\  \hline

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