COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 2924 Konkoly Observatory Budapest 15 August 1986 HU ISSN 0374 - 0676 UBV PHOTOMETRY OF YZ Cas The bright (V=5.66m), northern hemisphere eclipsing binary YZ Cas HD 4161, V0042+746 (1950.0), BS 192, BD+74d 27, 21 Cas, ADS 624A) lends itself favourably to differential photometry with small telescopes. When we started our joint observing program in 1983, only one published set of photoelectric measurements was known to us, namely the data of Kron (1939, 1942). Since Kron's observations, depicted in Figure 1 of Lacy (1981), were marred by night-to-night variations in the observational set-up (see Lacy, 1981), we decided to secure a new set of two-colour BV photometry (Johnson system), in order to facilitate the determination of the photometric elements of this eclipsing binary. All determinations of these elements up to that point by various authors (Kron, 1942; Kitamura, 1965; Cherepashchuk et al., 1968; Budding, 1973; Shulberg and Murnikova, 1974; Demircan, 1978; Kurutac, 1978; Mezzetti et al., 1980) have rested on Kron's data. In the meantime, an extensive set of observations in the four colours of the Utrecht photometric system has been published by de Landtsheer (1983) together with a thorough discussion of the physical properties of YZ Cas. We obtained 305 differential measurements in B and V as well as 135 differential observations in U, B and V (Johnson system) with two small telescopes. RL used an uncooled 1P21 photomultiplier tube along with a standard UBV filter set on his 52 cm Cassegrain reflector, while RD employed a commercial Starlight-1 photometer (EMI 9924A tube) with appropriate BV filters on a 35 cm Schmidt-Cassegrain type reflector. Each observation consists of at least three deflections in each colour and is corrected for differential extinction and transformed to the standard Johnson UBV system by the usual reduction procedures. Our data has been deposited in the I.A.U. archives of unpublished observations as file no. 194 (Breger, 1985). The primary comparison star used by both observers was HD 4382 (23 Cas: V=5.42, B-V=0.01 (97 obs.)) situated very close to the variable and of similar spectral type. HD 6163 served as check star, RL determined the brightness of the comparison and check star given in Table I from numerous differential observations with HD 3366. From our data, the time of minimum light of two primary and one secondary eclipse was determined employing the method of Kwee and van Woerden (1956). Table II contains the information on these minima, where the O-C_1 values refer to the elements JD_hel=2445561.4545 + 4.467224 * E (1) while the O-C_2 values are calculated from the elements of de Landtsheer (1983): JD_hel=2444632.2685 + 4.4672234 * E (2) No appreciable change of the period over the last 50 years is apparent. In Figure 1, we have plotted our V and B-V measurements according to the elements (1). The agreement of this light curve with the ones given by Kron (1939, Figure 2 of Lacy, 1981) and de Landtsheer (1983) is very good, as expected for this well detached binary system (see Figure 4 of Lacy, 1981). Nevertheless, the out-of-eclipse light curve shows more scatter than expected from the observational statistics of this bright star. A similar behaviour was noted by Kron (1939) and interpreted as nightly variation in the optical path. De Landtsheer (1983) also found larger scatter than expected (+-0.025m) in his data, and this fact was assigned to the atmospheric conditions at the observing site. We believe, that both these arguments are not sufficient to account for the scatter in our data, especially not for the observations of RL, which comprises the bulk of the current material, and a secondary source of variability remains as possible alternative. Either the comparison star (HD 4382=23 Cas) or one of the components of the binary system might show slight variability at the 0.03m level. In order to check these possibilities, RL observed the primary comparison star frequently against the check star HD 6163, finding no variation exceeding the observational scatter. This leaves a slight variability of one of the binaries components as most probable source of the excessive scatter, and the metallic line characteristics of the primary (B9.5 IVm, de Landtsheer, 1983) might point to this source. Unfortunately, the accuracy of our data does not lead to a positive result. More observations with a simultaneous two-channel photometer might clarify the question. An analysis of our data was not attempted, as the currently available photometric elements (Lacy, 1981; de Landtsheer, 1983) show excellent agreement and could hardly be enhanced with our data. Table I: BV data for the comparison and the check star Star V B-V n [mag] [mag] HD 4382 5.42 +0.01 97 HD 6163 6.82 +0.11 9 Table II: Times of minimum light of YZ Cas JDhel n Col Type O-C_1 O-C_2 Obs [d] [d] [d] 2445583.7863 +-0.0009 12 V p -.0043 +.0092 RL .7867 0.0004 12 B p -.0039 +.0096 2445621.748 0.005 26 V s -.014 -.010 RL .756 0.004 26 B s -.006 -.002 2445990.297 0.004 7 V p -.011 -.007 RD .305 0.006 7 B p -.003 +.001 [FIGURE 1] Acknowledgements: RD would like to express his gratitude towards the Swiss National Science Foundation for partial support of this investigation. R. DIETHELM Astronomisches Institut der Universitaet Basel Venusstrasse 7, CH-4102 Binningen, Switzerland R. D. LINES Lines Observatory, Mayer, Arizona USA 6030 North 17th Place, Phoenix, Arizona 85016, USA References: Breger, M.: 1985, Publ. Astron. Soc. Pacific 97, 85. [BIBCODE 1985PASP...97...85B ] Budding, E.: 1973, Astron. Astrophys. 22, 87. [BIBCODE 1973Ap&SS..22...87B ] Cherepashchuk, A. M., Goncharskii, A. V., Yagola, A. G.: 1968, Astron. Zh. 45, 1191. [BIBCODE 1968AZh....45.1191C ] Demircan, O.: 1978, Astrophys.Space Sci. 56, 389. [BIBCODE 1978Ap&SS..56..389D ] Kitamura, M.: 1965, Adv.Astron.Astrophys. 3, 27. Kron, G. E.: 1939, Lick. Obs. Bulletin 19, 59. [BIBCODE 1939LicOB..19...59K ] Kron, G. E.: 1942, Astrophys. J. 96, 173. [BIBCODE 1942ApJ....96..173K ] Kurutac, M.: 1978, Astrophys. Space Sci. 57, 71. [BIBCODE 1978Ap&SS..57...71K ] Kwee, K. K., van Woerden, H.: 1956, Bull. Astron. Inst. Neth. 13, 327. [BIBCODE 1956BAN....12..327K ] Lacy, C. H.: 1981, Astrophys. J. 251, 591. [BIBCODE 1981ApJ...251..591L ] de Landtsheer, A. C.: 1983, Astron. Astrophys. Suppl. Ser. 53, 161. [BIBCODE 1983A&AS...53..161D ] Mezzetti, M., Predolin, F., Giuricin, G., Mardirossian, F.: 1980, Astron. Astrophys. Suppl. Ser. 42, 15. [BIBCODE 1980A&AS...42...15M ] Shulberg, A. M., Murnikova, V. P.: 1974, Perem. Zvezdy 19, 421. [BIBCODE 1974PZ.....19..421S ] [DATAFILE 3]