COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3758 Konkoly Observatory Budapest 24 August 1992 HU ISSN 0374 - 0676 B and V photometry and the ephemeris of the W-UMa type star RS Serpentis RS Ser (HV 3073, Sp=F8, V=10.8) with coordinates alpha_1950 18h14m29s delta_1950 -13d04'.7 is listed in the GCVS (Kholopov et al., 1985) as a W-UMa type star with a 0.598140369 day period. According to the Rocznik Astronomiczny (Rudnicki, 1990), the star was not the subject of a published investigation since 1954. From 23 visually determined minima collected by GEOS observers (1982-1990) and from 3 minima taken from the literature (1915-1954), Walas (1991) derived the following ephemeris: Min I JJ_Hel 2447355.4509 + 0.5981434 (1) +/- 16 +/- 2 (95% of level confidence) This paper reports new photoelectric measurements obtained by M. Dumont, E. Joffrin, A. Kucinskas, J. Remis and T. Vezauskas with the 76-cm cassegrain telescope and the Geneva photometer, during a run organized by the Palais de la Decouverte. The B and the V filter values of the Geneva system and the B-V ones have been converted into the Johnson and Morgan system using the formula suggested by Meylan (1981). The correlation to standard was ensured by frequent observations of standard stars selected from the Geneva catalogue which were used to compute the first and second order atmospheric coefficients for each night. The complete method of reduction is described by Dumont (1983). Although the relative accuracy obtained with the standards photometry method is rather low compared with the differential one, it allows more measurements on the variable stars and computation of the B and V magnitudes from the same set of standards. The observations were carried out during 5 nights between August 9 and August 18, 1991. All the observations are listed in Table 1. For all the observations, the air-mass was larger than 2, which is an unavoidable consequence of the southern declination of RS Ser. Because of that, there are only 2 decimals in the listed V values. Despite the fact that the air-masses were so high, our results show a fair degree of consistency, which reflects the quality of this high-alpine site. Table 1 V and B-V measurements of RS Ser JD (Hel) V B-V Air-Mass 48478.4206 11.18 0.70 2.17 48478.4226 11.20 0.71 2.19 48478.4265 11.23 0.71 2.22 48478.4289 11.25 0.72 2.23 48478.4310 11.26 0.72 2.25 48478.4348 11.31 0.72 2.29 48478.4362 11.32 0.71 2.31 48478.4390 11.35 0.71 2.33 48478.4417 11.40 0.73 2.37 48478.4438 11.41 0.71 2.40 48478.4459 11.43 0.70 2.42 48478.4501 11.48 0.68 2.48 48478.4522 11.52 0.70 2.51 48478.4556 11.53 0.69 2.55 48478.4605 11.65 0.69 2.63 48478.4626 11.69 0.69 2.66 48478.4653 11.67 0.69 2.71 48478.4674 11.65 0.76 2.75 48478.4723 11.58 0.74 2.86 48478.4744 11. 5 0.70 2.92 48478.4765 11.53 0.71 2.97 48478.4813 11.46 0.73 3.09 48478.4841 11.48 0.69 3.16 48479.3813 11.58 0.79 2.00 48479.3837 11.54 0.80 2.01 48479.3861 11.49 0.81 2.01 48482.3891 11.21 0.78 2.10 48484.4344 11.60 0.75 2.49 48484.4358 11.61 0.73 2.51 48484.4372 11.61 0.74 2.53 48484.4385 11.64 0.72 2.55 48484.4448 11.68 0.74 2.65 48484.4462 11.69 0.74 2.68 48484.4476 11.68 0.72 2.70 48484.4490 11.68 0.74 2.73 48484.4524 11.62 0.75 2.80 48484.4538 11.59 0.73 2.83 48484.4552 11.57 0.74 2.86 48484.4566 11.56 0.75 2.89 48487.4335 11.68 0.68 2.60 48487.4355 11.67 0.69 2.64 48487.4369 11.68 0.72 2.66 48487.4383 11.67 0.72 2.69 48487.4418 11.65 0.71 2.75 48487.4432 11.61 0.71 2.78 48487.4446 11.57 0.75 2.81 48487.4480 11.51 0.74 2.89 48487.4501 11.47 0.76 2.94 48487.4515 11.46 0.75 2.97 48487.4557 11.39 0.74 3.08 48487.4571 11.37 0.72 3.12 48487.4585 11.35 0.71 3.16 48487.4598 11.34 0.72 3.20 48487.4633 11.27 0.73 3.31 48487.4647 11.24 0.72 3.35 48487.4661 11.24 0.72 3.40 48487.4675 11.24 0.72 3.45 48487.4710 11.14 0.74 3.58 48487.4723 11.12 0.74 3.64 48487.4737 11.11 0.72 3.70 48487.4751 11.13 0.70 3.76 The V-Phase diagram in Figure 1 shows that only the secondary eclipse has been covered. The mean B-V index is equal to 0.72. Two secondary eclipses have been observed, and a part of the ascending branch for the primary one. The method of Kwee and Van Woerden (1956) for computing the epochs of minima of eclipsing variables was applied using phase intervals of 0.425 to 0.575 (secondary eclipse). Table 2 shows the photoelectric O-C's referring to the revised ephemeris (1). [FIGURE 1] Fig-1: V-Phase diagram of RS Ser according to ephemeris (1) An analysis of these data shows that the light parameters mentioned for this star in the last edition of GCVS (Kholopov et al., 1985) are erroneous. As was suggested in a GEOS Circular (Walas, 1991), the minima are deeper than previously stated. The net difference of about 0.1 magnitude noticed by all the visual observers leads to the following light parameters for the two eclipses: Min I : 11.8 V ? (GCVS 85 : 11.5 V) Min II: 11.69 +/- 0.01 (V) (GCVS 85 : 11.4 V) 12.40 +/- 0.02 (B) Table 2 Photoelectric times of minima Date Hel. Julian Day O-C(1) 09.08.1991 24448478.4649 -0.0002 15.08.1991 24448484.4441 -0.0025 The large eclipse depth suspected from the slope of the ascending and descending branches (near 0.8 magnitude if the star was an EW-type), the duration of the observed eclipse, the net difference between the primary and the secondary ones, make us doubt about the EW nature of RS Ser. We tend to believe that RS Ser may be an EA or an EB-type star. RS Ser deserves further attention. A complete light curve, based on differential data obtained at an appropriately located site, will allow a more accurate determination of the light elements and the consequent computation of a synthetic solution for the system. These efforts are planned at GEOS in the next months. O. WALAS ^1 M. DUMONT ^1,2 J. REMIS ^1 1 GEOS 3, Promenade Venezia F-78000 VERSAILLES 2 Palais de la Decouverte Avenue Franklin D. Roosevelt F-75008 PARIS References: Dumont, M. 1983, GEOS Circular RR 7. Kholopov, P.N. et al. 1985, General Catalogue of Variable Stars. Kwee, K.K., Van Woerden, H. 1956, B.A.N 12, 327. [BIBCODE 1956BAN....12..327K ] Meylan, G., Hauck, B. 1981, Astron. Astrophys. Suppl. Ser. 46, 281-303. [BIBCODE 1981A&AS...46..281M ] Rudnicki, K. 1990, Rocznik Astronomiczny SAC 62, Krakow. [BIBCODE 1990raok.book.....R ] Walas, O. 1991, GEOS Circular EB 19.