COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1826 Konkoly Observatory Budapest 1980 August 4 HU ISSN 0374-0676 PHOTOELECTRIC PHOTOMETRY OF THE BRIGHT METALLIC-LINE ECLIPSING BINARY delta CAPRICORNI delta Capricorni (HR 8322; HD 207098; ADS 15314) is one of the nearest (d ~ 15pc) and brightest (V_max= +2.83) eclipsing binary systems, consisting of a metallic-line A7-type primary and a cooler, less massive component. Although it has been known to be a variable star for about 25 years, little attention has been paid to it, and no light curve has yet been published. Spectrographic studies of the system have been made by Crump (1921), Stewart (1958), and Batten (1961) where only the spectrum of the Am star has been seen. The light variability of delta Cap was discovered by Eggen (1956) when the primary eclipse with a depth of about 0.16 mag in V was observed. Subsequent photoelectric observations made by Wood and Lampert (1963) confirm Eggen's result and reveal possible variations in the depth of the primary eclipse. They suggest that this may be due to variations in the light of the comparison star. Photoelectric observations of delta Cap were obtained on 39 nights from September 1977 through October 1978 at Biruni Observatory using the 51 cm reflector and at Villanova University Observatory using the 38 cm reflector. The Biruni telescope is equipped with an RCA 4509 photomultiplier tube and a chart recorder was used to record the observations. The photoelectric photometer employed with the Villanova telescope is equipped with a thermoelectrically cooled (to -10 C) RCA C37034A gallium-arsenide photocell and a microprocessor-controlled integrating system. Fairs of matched intermediate- and narrow band interference filters centered near the wave-length of the Balmer H_alpha line (lambda 6563) were used at both observatories. The H_alpha filter pairs are similar to those used by [FIGURE 1] FIGURE 1: The lambda 6585 light curve and alpha-index of delta Capricorni plotted against orbital phase. Circles (o) are Biruni observations, crosses (x) are Villanova observations. Baliunas, Ciccone and Guinan (1975) in the definition of the Villanova H_alpha system.The H_alpha intermediate bandpass filter is centered at 6585A with a FWHM = 280A It is broad enough to be little affected by the presence of the H_alpha feature within the bandpass. Additional observations were obtained at Biruni Observatory using a Stromgren u filter and at Villanova using an intermediate-band filter centered near 7790A, with a FWHM = 180A. The comparison star was gamma Cap while kappa Cap and mu Cap served as check stars. The observing sequence was the standard pattern of sky-comparison-variable-comparison-sky with each observation lasting about 50 seconds. The effects of differential atmospheric extinction were removed, and 2 point normals were formed. In Fig. 1, differential magnitudes in the H_alpha intermediate bandpass are plotted against the phase, computed from the ephemeris given by Eq. 1 where zero phase corresponds to the time of mid-primary eclipse. Also shown in Fig. 1 is the alpha-index, defined in the usual way (Dorren et. al, 1980) which yields a measure of the net strength of the H_alpha line. As shown in the figure, the light curve is well defined and shows two minima of unequal depths and two rounded maxima. The secondary minimum appears to occur close to 0.50 phase in agreement with the circular orbit indicated spectroscopically by Batten. The depths of primary and secondary minima are about 0.18 mag and 0.05 mag, respectively. The depths of primary and secondary minima for the u and the lambda 7790 bandpasses are essentially identical with the values given above for lambda 6585. It is clear that there is considerable scatter in the observations. No significant variations were found between the comparison star and the check stars above the level of +-0.008 mag, however, so that the observed scatter in the light curve appears intrinsic to delta Cap. Light variations on a time scale of several hours also appear in the data. As shown in the figure, the scatter in the alpha-index is relatively large. A peculiar phase-dependent variation appears to be present with (Delta alpha) having the smallest (negative) values from 0.1 to 0.5 phases and its largest value from about 0.7 to 0.9 phase. This behaviour indicates that the net H_alpha (absorption) line strengh is weakest from 0.1 to 0.5 phase and strongest at 0.7 to 0.9 phase. A small decrease in the alpha-index is expected at primary eclipse when the A7m star with its strong H_alpha line is partially eclipsed by a cooler star with relatively weak hydrogen lines. Although the effect of the eclipse can be seen in the alpha-index from 0.95 to 0.05 phases, the behaviour of the alpha-index described above is anomalous and could indicate the presence of a gas stream or a hot (or cool) spot on the surface of the hotter component. Similar behaviour in H_beta and H_alpha has been observed by Guinan (1971) for the 0.667 day eclipsing binary V1010 Oph. A more refined light ephemeris was determined by combining the two previous times of primary minimum given by Eggen and by wood and Lampert with our own timings to obtain: T (MIN.I) = HJD 2435656.913+1d0227688.E (1) +-2 +-3 There is no evidence of any significant change in the apparent period. A preliminary analysis o the observations indicates an orbital inclination of i ~ 67deg, with the primary eclipse a partial transit. It would appear that the system is composed of an A7m primary and a cooler (~4700d K) secondary near its Roche lobe. A detailed analysis of the light curves using the Wilson-Devinney program will be published later. J.D. DORREN E.F. GUINAN R.R. DEL CONTE M.J. SIAH G.P. Mc COOK O.L. LUPIE Biruni Observatory M.J. ACIERNO K.N. YOUNG Shiraz University Department of Astronomy Shiraz, Iran Villanova University, Villanova Pennsylvania 19085 References: Baliunas, S.L., Ciccone, M.A. and Guinan, E.F 1975, Pub.Astron. Soc.Pac. 87, 969 [BIBCODE 1975PASP...87..969B ] Batten, A.H., 1961, Pub. Dominion Astrophys. Obs. 11, 395 [BIBCODE 1961PDAO...11..395B ] Crump, C.C., 1921, Astrophys. J. 54, 127 [BIBCODE 1921ApJ....54..127C ] Dorren, J.D., Guinan, E.F., Hart, M.K. and Riazi, N., 1980, I.B.V.S. No. 1745 Eggen, O.J., 1956., Pub.Astron.Soc.Pac. 68, 542 [BIBCODE 1956PASP...68..541E ] Guinan, E.F., 1971, Bull.Am.Astron.Soc., 3, 444 [BIBCODE 1971BAAS....3R.444G ] Stewart, M., 1958, J.Roy.Astron.Soc.Can., 52, 11 [BIBCODE 1958JRASC..52...11S ] Wood, F.B. and Lampert, G., 1963, Pub.Astron.Soc.Pac., 75, 281 [BIBCODE 1963PASP...75..281W ]