COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3342 Konkoly Observatory Budapest 22 June 1989 HU ISSN 0374 - 0676 PHOTOMETRIC VARIATION OF ZETA AURIGAE Zeta Aur is a prototype for a group of long period spectroscopic (and eclipsing) binary systems that consists of a K-type supergiant and a B-type main sequence companion. Before and after the eclipses in these systems, the light of the hot secondaries shine through the extended atmospheres of K-type supergiants. The Zeta Aur system has been studied in detail since Harper (1924) first discovered it to be an eclipsing binary. The primary is a K3 or K4 Ib or II type star (Wright 1970), and the secondary is a B7 V star (Wright 1970; Faraggiana and Hack 1980). For a more complete early bibliography on the system see Wilson (1960), Wright (1970), and Hack and Stickland (1987). To investigate the nature of the photometric variations outside the eclipses of Zeta Aur, we have observed the system with UBV filters attached to the photoelectric photometer of the 30 cm Maksutov Telecope of Ankara University Observatory. Lambda Aur was taken as comparison star. A sequence; comparison-comparison sky-variable-variable sky-variable-comparison-comparison sky in each of the three filters was followed to secure on differential observation in the sense of variable minus comparison. Thus, altogether 56 differential magnitude measurements in each bandpass were obtained on 23 nights in 1982 and 1983. The individual differential magnitudes (a few measurements per night) were corrected for differential atmospheric extinction and the times of measurement were reduced to heliocentric Julian date. The nightly mean observations were listed in Table 1 and plotted in Figure 1 against the heliocentric Julian date. [FIGURE 1] Figure 1. The nightly mean UBV observations of Zeta Aurigae. Although we have no observations between HJD 2445069 and 2445224, a periodic light variation with an amplitude around 0.1m in all three bandpasses is seen in Figure 1. It seems the amplitude of the variation increases towards the shorter wavelengths. The 1982 eclipse of the secondary by the supergiant occured on 9 th August but unfortunately we have no observations during the eclipse. A preliminary period analysis of our observations for the low amplitude long term variation yields two probable periods: 151 days and its double 302 days. The true period is more probably around 300 days. Table 1. The nightly mean UBV observations of Zeta Aurigae. HJD DV0 DB0 DU0 45057.2360 -0.977 -0.298 +0.000 45068.3055 -0.933 -0.241 +0.094 45224.5912 -0.969 -0.321 -0.065 45265.6013 -0.991 -0.328 -0.054 45267.4122 -0.996 -0.341 -0.093 45268.4956 -0.996 -0.333 -0.079 45271.3300 -1.007 -0.382 -0.164 45283.4022 -1.004 -0.360 -0.125 45286.3371 -1.010 -0.359 -0.110 45289.3310 -1.040 -0.398 -0.121 45298.4594 -1.004 -0.350 -0.082 45299.3175 -1.083 -0.412 -0.136 45300.4175 -1.008 -0.360 -0.101 45302.3812 -1.014 -0.355 -0.107 45311.2721 -1.012 -0.358 -0.113 45313.2580 -0.984 -0.380 -0.082 45320.2965 -0.366 -0.087 45321.2515 -1.019 -0.370 -0.111 45326.2642 -1.024 -0.364 -0.116 45327.2850 -0.981 -0.342 -0.103 45360.4640 -0.941 -0.287 -0.015 45405.2723 -0.954 -0.290 -0.031 Although Hutchings and Wright (1971) suggested that such light variations in these systems should come from the H_alpha emission region with a radius of 150 R_sun, surrounding the Be components, we believe that a pulsation of cool supergiant in these systems is involved in the observed photometric variations. The K supergiant component of Zeta Aur is probably a semiregular low amplitude red variable, like alpha Herculis, with a pulsation period greater than 100 days. OSMAN DEMIRCAN ETHEM DERMAN CEMAL AYDIN AYVUR AKALIN Dept. of Astronomy and Space Sciences, University of Ankara, 06100 Tandokan, Ankara, Turkey. References: Faraggiana, R. and Hack, M.: 1980, Proc. Second European IUE Conf., Tubingen, Germany (ESA SP-157). [BIBCODE 1980ESASP.157..223F ] Hack, M. and Stickland, D.: 1987, in Scientific Accomplishments of the IUE (ed. Y. Kondo), D. Reidel Publ. Co., p 445. [BIBCODE 1987ASSL..129..445H ] Harper, W. E.: 1924, Publ. Dom. Ap. Obs. Victoria, 3. 151. [BIBCODE 1924PDAO....3..151H ] Hutchings, J. B. and Wright, K. O.: 1971, MNRAS, 155, 203. [BIBCODE 1971MNRAS.155..203H ] Wilson, O. C.: 1960, in Stellar Atmospheres (ed. J. L. Greenstein), Chicago Univ. Press, p 436. [BIBCODE 1960stat.conf..436W ] Wright, K. O.: 1970, Vistas Astr. 12, 147. [BIBCODE 1970VA.....12..147W ]