COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3861 Konkoly Observatory Budapest 31 March 1993 HU ISSN 0374 - 0676 NEW PHOTOELECTRIC MINIMA TIMES OF V566 OPHIUCHI AND ITS PERIOD STUDY The binary system V566 Oph (HD163611, BD+5d 3547, Sp:F2/4Vn) is a short period eclipsing variable of W UMa-type. It is one of the best observed W UMa-type systems with many light curve analyses and minima times. V566 Oph has been observed photoelectrically with a two-beam, multi-mode, nebular-stellar photometer attached to the 48-inch Cassegrain reflector at the Kryonerion Astronomical Station, Greece. The stars BD+4d 3553 and BD+4d 3556 were used for comparison and checking, respectively. Reduction of the observations has been made as usual (Hardie, 1962) and the passbands of the B and V filters used are in close accordance with the standard ones. From our observations of V566 Oph four new minima times were derived using Kwee and Van Woerden's (1956) method and these are the mean values of B and V. They are given in Table 1; the successive columns of which present: Hel. JD, (O-C)_I, E_I and (O-C)_II, E_II. In the residuals (O-C)_I and (O-C)_II the C's have been calculated using the following ephemeris formulae, respectively. (I): MinI=2435245.5440+0.40964101d x E (1) (due to Binnendijk, 1959) (II): MinI=2440047.3478+0.40964600d x E (2) (due to Seeds & Dawson, 1985) Table 1 New Photoelectric Minima Times of V566 Ophiuchi Hel. JD. (O-C)_I E_I (O-C)_II E_II 2440000+ 7298.4876 0.0762 29423 -0.0040 17701 7299.5138 0.0783 29425.5 -0.0019 17703.5 7300.5383 0.0787 29428 -0.0015 17706 7301.5607 0.0770 29430.5 -0.0032 17708.5 From all minima times of V566 Oph found in the literature and our new ones given in Table 1 and for linear least squares fitting, formulae (1) and (2) become, respectively: MinI=2435245.2258 + 0.40964923d x E (3) +-0.0028 +-0.00000004 and Min I=2440047.5348 + 0.40964183d x E (4) +-0.0025 +-0.00000003 while for quadratic least squares fitting we get: Min I=2435246.6389+0.40958120d x E+8.13 x 10^-10 x E^2 (5) and Min I=2440049.0395+0.40956939d x E+8.65 x 10^-10 x E2 (6) respectively. [FIGURE 1] Figure 1. The O-C diagram of V566 Oph according to Binnendijk's (1959) ephemeris. Quadratic fitting has been applied to all minima times and linear (- - -) after Hel. JD 2442000. [FIGURE 2] Figure 2. Same as Fig. 1, but according to Seeds & Dawson's (1985) ephemeris formula. [FIGURE 3] Figure 3. The O-C diagram of V566 Oph according to Seeds & Dawson's ephemeris. Quadratic fitting has been separately applied to primary (+) and secondary (*) photoelectric minima times only. [FIGURE 4] Figure 4. Sames as Fig. 3, but the visual minima are also included. Visual primaries are denoted by [diamond], while secondaries by [square]. Moreover, if the visual minima times are not taken into account, formulae (1) and (2) for linear least squares fitting become, respectively: Min I=2435245.2881 + 0.40964769d x E (7) +-0.0017 +-0.00000004 and Min I=2440047.5986 + 0.40964025d x E (8) +-0.0013 +-0.00000003 while for quadratic one we get: Min I=2435245.7911+0.40957370d x E+9.05 x 10^-10 E^2 (9) and Min I=2440049.2109+0.40956089d x E+9.70 x 10^-10 x E^2 ( 10) From the foregoing linear ephemeris formulae (3), (7) the period of V566 Oph given by Binnendijk (P=0.40964101d) seems to have increased with a rate which is greater if all minima times are considered. While formulae (4) and (8) indicate that the period given by Seeds and Dawson (P=0.40964600d) has decreased with a rate which is greater if the visual minima are not taken into account. Of course, neither the linear fitting, nor the quadratic one is suitable for all minima times of V566 Oph. (For a new method see Kalimeris et al., 1993). In Figures 1 and 2 quadratic fitting has been applied to photoelectric minima times only using Binnendijk's (1959) and Seeds & Dawson's (1985) ephemeris formulae, respectively. From these, one can notice that after Hel. JD 2442000 the O-C values could be approached by a linear fitting. Doing so, formulae (1) and (2) are improved to: Min I=2435244.9972 + 0.40965433d x E (11) +-0.0015 +-0.00000004 and Min I=2440047.2991 + 0.40964711d x E (12) +-0.0010 +-0.00000003 respectively, which show that the period of V566 Oph continues increasing; but more data are needed to examine its future behaviour. Moreover, since the number of both primary and secondary minima times was large enough, a separate O-C diagram has been drawn for each one of them. The results are presented in Figures 3 and 4, which are based on Seeds & Dawson's (1985) ephemeris. The consistency of the two curves, corresponding to primary (+) and secondary minima (*) respectively, is much better in Figure 3 than in Figure 4, where the visual minima ([diamond] for primaries and [square] for secondaries) are also included. H. ROVITHIS-LIVANIOU P. G. NIARCHOS Section of Astrophysics Astronomy and Mechanics Athens University GR 157 84 Zografos Athens, Greece P. ROVITHIS Astronomical Institute National Observatory of Athens, P.O. Box 20048 Athens, Greece References: Binnendijk, L., 1959, Astron. J., 64, 65 [BIBCODE 1959AJ.....64...65B ] Hardie, R. H., 1962, in W.A. Hiltner (ed.), Stars and Stellar Systems Vol. II, Astronomical Techniques, The University of Chicago Press, Chicago Kalimeris, A., Rovithis-Livaniou, H. and Rovithis, P., 1993, Astron., Astrophys. (in press) Kwee, K. K. and Van Woerden, H., 1956, Bull. Astron. Inst. Neth., 12, 327 [BIBCODE 1956BAN....12..327K ] Seeds, M. A. and Dawson, D. W., 1985, IBVS, No. 2836