COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3485 Konkoly Observatory Budapest 26 June 1990 HU ISSN 0374 - 0676 PERIOD VARIATIONS OF SS ARIETIS The aim of this work is to provide an argument for the need of to continuing to monitor the minima of the eclipsing binary system SS Ari by photoelectric observations. The variability of this W UMa-type variable was found by Hoffmeister (1934). The history of the first observations is presented by Kaluzny, Pojmanski (1984). The observations of SS Ari were performed in the Astronomical Observatory of Jagiellonian University at Cracow using a 50 cm Cassegrain telescope with a single channel photometer and, in the last two seasons, at Mt. Suhora Astronomical Observatory of the Cracow Pedagogical University using a 60 cm Cassegrain telescope coupled with a two-channel photometer. The times of minima determined from our observations are presented in Table I. They were found as the minimum of a parabola fitted into the observational points by means of least-squares method. Table I. New photoelectric minima of SS Ari. JD hel. m.e. Type Colour Observer JD hel. m.e. Type Colour Observer 2440000.+ 0.0001 240000.+ 0.0001 2727.4083 10 I B MKW 6716.4360 10 II V MKW 2727.4088 9 I V " 6745.4636 8 I V " 2758.2638 8 I V " 6761.2955 5 I B " 2758.2645 12 I B " 7143.3294 4 I V " 2759.2790 8 II V " 7439.4952 3 II V BZ 2759.2792 9 II B " 7439.4960 2 II B " 3455.3481 6 I V " 7439.4962 5 II V MKW 3815.4581 15 I V " 7444.5695 6 I V " 3927.3091 3 II V " 7449.4414 2 I V BZ 4146.5426 10 II B " 7449.4416 2 I B " 4469.5074 6 I V " 7449.4420 5 I V MKW 4605.3103 6 II V " 7450.4569 5 II V " 4642.2567 11 II V " 7452.4872 5 II V " 4823.5267 10 I V " 7561.2904 6 II V " 5295.2840 12 I V " 7823.5573 5 II V " 5593.4817 3 II V " 7823.5580 6 II V BZ 5609.5174 5 I V " 7828.4289 5 II V MKW 5698.2264 7 II V " 7834.3148 4 I V " 6714.6062 6 I V " 7848.3226 5 II V " MKW - M. Kurpinska-Winiarska, Cracow, BZ - B. Zakrzewski, Suhora From Table 1, together with the other 161 minima (26 phe, 130 vis, 5 pg) collected from the literature up to Dec. 1989, the average period of SS Ari was found to be 0.4059909d. A complete list of minima is available on request. Figure 1 presents the O-C diagram of the epochs of minimum light based on the ephemeris Min. I = JD hel. 2444469.5074 + 0.4059909d E +/-6 +/-6 where one of the minima from table I is adopted as an epoch. [FIGURE 1] Figure 1. Period variations of SS Arietis It is clearly visible that the period of SS Ari is variable. However, the interpretation of the character of this variability affords difficulties. At first sight the O-C diagram has a sine - like shape (Kaluzny, Pojmanski, 1984) resulting from the positions of minima observed before 1965, ie. 7 visual minima (3 normal ones included) and 2 photographic ones. Some estimation of the accuracy of these minima can be obtained from the 23 individual minima, marked in Fig. 1 as small dots, determined by Odynskaya (1943) from Tsessevitch's observations. The same observations were used by Odynskaya in the determination of the normal minimum JD hel. 2430945.329. As can be seen, the scattering attains the value of +/-0.05 d. Similarly the visual minima after 1965 reveal a dispersion of the same order around the photoelectric ones. This probably results from the application of Argelander's method to the observations of a star with a small amplitude (about 0.5 mag in V) and a short period, as well as from a small number of estimates of brightness, which has not exceeded 10 for 50% of visual minima. So, in further discussion, we take only the photoelectric minima. This part of the O-C diagram can be fitted by a sine-like curve (solid line), as well as a parabola (dashed line). For this calculation the non-linear least squares method's algorithm of Bevington (1969) was used. The best-fit sine curve was found under the condition that the sine period is longer than 30 years and e = 0, as Fig. 1 indicates. The derived ephemeris of times of minimum is. Min. I = JD hel. 2444469.4732 + 0.4059909d E + 0.0417d sin(0.000358*tmin + 4.34) +/-14 +/-6 +/-14 +/-8 +/-.34 and gives a period of about 49 years. The parabolic ephemeris gives Min. I = JD hel. 2444469.5065 + 0.4059504d E - 0.338d 10^-9 E^2 +/-4 +/-5 +/-8 The dispersions of the two fits are of the same order, of +/-0.001d. However, their comparison may be treated as qualitative only, because of a different number of unknowns for each of the fits. All data presented in Fig. 1 fit well the sine curve, but also, a parabolic fit of minima before JD 2435000 is almost acceptable (see above - visual minima accuracy), if the O-C values are diminished by 1/2P. The obtained results do not definitely settle the problem of geometrical interpretation of O-C diagram. The representation of the O-C diagram in the form of fragments of straight lines separated by fragments of parabolae as suggested by Kreiner (1977) does not reproduce the diagram. A computation indicates a continuous change of the straight lines' slope. It is obvious that different geometrical interpretations of the O-C diagram for SS Ari give completely different physical mechanisms underlying the period variations. Future photoelectric observations of times of minima would throw some light on this problem. M. KURPINSKA-WINIARSKA B. ZAKRZEWSKI Astronomical Observatory Institute of Physics of The Jagiellonian University of the Pedagogical University ul. Orla 171, 30-244 Krakow Podchorazych 2, 30-084 Krakow POLAND POLAND References: Bevington, F.R., 1969, "Data Reduction and Error Analysis for the Physical Sciences", ed. McGraw-Hill, Inc., New York [BIBCODE 1969drea.book.....B ] Hoffmeister, C., 1934, A.N. 253, 193 [BIBCODE 1934AN....253..195H ] Kaluzny, J., Pojmanski G., 1984, Acta Astr. 34, 445 [BIBCODE 1984AcA....34..445K ] Kreiner J.M., 1977, in "The interaction of Variable Stars with their Environment" ed. Kippenhahn et al " p. 393 [BIBCODE 1977ivsw.conf..393K ] Odynskaya O.K., 1948, Perem. Zvezdy, 60, 316