COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 2978 Konkoly Observatory Budapest 26 January 1987 HU ISSN 0374 - 0676 AT PEGASI: PHOTOELECTRIC OBSERVATIONS AND LIGHT ELEMENTS The only existing photoelectric light curve of the eclipsing binary AT Peg (HD 210892 = BD+07d4824) was given by Cristaldi and Walter (1963). It is reanalyzed by Giuricin et al. (1981). Several authors observed the system visually and photographically. The published times of minima are listed in Table I. The spectroscopic orbital elements are studied by Hill and Barnes (1972). Margrave (1981) investigated the period changes of the system by combining his photoelectric times of minima with the other photoelectric ones since 1969. He concluded that the period is decreasing. Moreover, this decreasing has been accelerated since 1975 which fits quadratic light elements. The system was observed photoelectrically at Ege University Observatory on 13 nights during the observational seasons of 1983, 1984, 1985 and 1986. The observations were made in blue and yellow colours using the 48 cm Cassegrain telescope and photoelectric photometer equipped with an unrefrigerated photomultiplier tube EMI 9781A and Johnson's standard B, V filters. A total of 529 and 396 points were obtained in B and V colours, respectively. BD+07d4818 was used as the comparison, and BD+08d4800 as the check star. The comparison star showed no significant variations against the check star. The extinction coefficients were determined for each night from observations of the comparison star. The differential observations (variable minus comparison) were corrected for the atmospheric extinction. Four primary and two secondary times of minima were obtained and included in Table I. Table I. Times of minima of AT Pegasi. JD Hel. Min Method W E (O-C) Ref. 2433504.526 I vis 1 -6050 -0.060 1 504.523 I vis 1 -6050 -0.063 1 558.387 I vis 1 -6003 -0.066 1 888.463 I vis 1 -5715 -0.061 1 34272.406 I vis 1 -5380 -0.056 2 303.350 I via 1 -5353 -0.056 2 35019.666 I pg 2 -4728 -0.041 3 034.572 I pg 2 -4715 -0.034 3 097.598 I pg 2 -4660 -0.043 3 332.541 I vis 1 -4455 -0.047 4 332.549 I vis 1 -4455 -0.039 4 370.363 I vis 1 -4422 -0.045 4 370.365 I vis 1 -4422 -0.043 4 388.710 I pg 2 -4406 -0.039 3 726.810 I pg 2 -4111 -0.030 3 36085.527 I vis 1 -3798 -0.037 4 085.528 I vis 1 -3798 -0.036 4 100.434 I vis 1 -3785 -0.029 4 108.447 I vis 1 -3778 -0.038 4 108.455 I vis 1 -3778 -0.030 4 108.460 I vis 1 -3778 -0.025 4 37175.478 I vis 1 -2847 -0.010 5 174.471 I vis 1 -2847 -0.017 6 497.5211 I pe 3 -2566 -0.0155 7 544.507 I vis 1 -2525 -0.019 5 544.515 I vis 1 -2525 -0.011 5 872.315 I vis 1 -2239 0.010 8 872.317 I vis 1 -2239 0.012 8 872.321 I vis 1 -2239 0.016 8 872.318 I vis 1 -2239 0.013 8 873.480 I vis 1 -2238 0.029 9 904.402 I vis 1 -2211 0.006 8 37911.287 I vis 1 -2205 0.015 8 38226.464 I vis 1 -1930 0.019 10 234.468 I vis 1 -1923 0.001 10 288.348 I vis 1 -1876 0.015 8 288.347 I vis 1 -1876 0.014 8 319.289 1 pg 2 -1849 0.012 10 642.458 I pg 2 -1567 -0.015 11 940.447 I vis 1 -1307 -0.007 12 940.454 I vis 1 -1307 0.000 12 39057.353 I vis 1 -1205 -0.001 12 356.480 I vis 1 -944 -0.002 6 356.485 I vis 1 -944 0.003 6 356.486 I vis 1 -944 0.004 6 387.406 I vis 1 -917 -0.020 13 685.443 I vis 1 -657 0.036 14 40407.438 I pe(V) 3 -27 -0.001 15 438.383 I pe(V) 3 0 0.000 15 477.326 I vie 1 34 -0.024 16 493.394 I pe(V) 3 48 -0.001 17 877.3368 I pe(V) 3 383 0.0044 18 877.3372 I pe(V) 3 383 0.0048 18 41576.446 I vis 1 993 0.004 16 599.378 I vis 1 1013 0.014 16 661.2728 I pe(B,V) 3 1067 0.0203 19 42661.8136 I pe(V) 3 1940 0.0315 20 712.2435 I pe(V) 3 1984 0.0338 21 43728.8093 I pe(V) 3 2871 0.0249 20 44089.8270 I pe(V) 3 3186 0.0267 20 128.7925 I pe(V) 3 3220 0.0255 20 136.8149 I pe(V) 3 3227 0.0253 20 442.8188 I pe(V) 3 3494 0.0253 22 520.7515 I pe(V) 3 3562 0.0244 22 826.7553 I pe(V) 3 3829 0.0243 23 45219.8562 I pe(V) 3 4172 0.0191 24 615.2538 I pe(B) 3 4517 0.0184 25 957.360 II pe(B) - 4815.5 0.019 25 46000.3358 I pe(B,V) 3 4853 0.0169 26 298.3155 I pe(B,V) 3 5113 0.0152 26 315.5062 I pe(B,V) 3 5128 0.0147 26 334.419 II pe(B,V) - 5144.5 0.017 26 References to Table I: 1. Domke, K. and Pohl, E.: 1952, Astron. Nachr. 281, 113. 2. Pohl, E.: 1954, Astron. Nachr. 282, 235. 3. Whitney, D.S.: 1957, Astron. J. 62, 371. [BIBCODE 1957AJ.....62..371W ] 4. Rudolph, R.: 1959, Astron. Nachr 285, 161. 5. Flin, P., Slowik,A.: 1967, Acta Astron. 17, 59. [BIBCODE 1967AcA....17...59F ] 6. Czerlunczakiewicz, D., Flin, P.: 1968, Acta Astron. 18, 331. [BIBCODE 1968AcA....18..331C ] 7. Cristaldi, S., Walter, K.: 1963, Astron. Nachr. 287, 103. 8. Pohl, E., Kizilirmak, A.: 1964, Astron.Nachr. 290, 105. 9. Oburka, O.: 1964a, Bull. Astron. Inst. Czech. 15, 26. [BIBCODE 1964BAICz..15...26O ] 10. Oburka, O.: 1964b, Bull. Astron. Inst.Czech. 15, 250. [BIBCODE 1964BAICz..15..250O ] 11. Oburka, O.: 1965, Bull. Astron. Inst. Czech. 16, 212. [BIBCODE 1965BAICz..16..212O ] 12. Braune, W., Hubscher, J.: 1967, Astron. Nachr. 290, 105. [BIBCODE 1967AN....290..105B ] 13. Braune, W., Hubscher, J., Mundry, E.: 1970, Astron. Nachr. 292, 185. [BIBCODE 1970AN....292..185B ] 14. Kizilirmak, A., Pohl, E.: 1968, Astron. Nachr. 291, 113. 15. Pohl, E., Kizilirmak, A.: 1970, Inform. Bull. Var. Stars, No. 456. 16. Braune, W., Mundry, E.: 1973, Astron. Nachr. 294, 225. [BIBCODE 1973AN....294..225B ] 17. Kizilirmak, A.: 1971, Publ. Ege Univ. Obs. No. 11-16. 18. Kizilirmak, A., Pohl, E.: 1971, Inform. Bull. Var. Stars, No. 530. 19. Kizilirmak, A., Pohl, E.: 1974, Inform. Bull. Var. Stars, No. 937. 20. Margrave, T.E.: 1980, Inform. Bull. Var. Stars, No. 1869. 21. Pohl, E., Kizilirmak, A.: 1976, Inform. Bull. Var. Stars, No. 1163. 22. Margrave, T.E.: 1981, Inform. Bull. Var. Stars, No. 1930. 23. Margrave, T.E.: 1982, Inform. Bull. Var. Stars, No. 2086. 24. Margrave, T.E.: 1983, Inform. Bull. Var. Stars, No. 2292. 25. Pohl, E., Tunca, Z., Gulmen, O., Evren, S.: 1985, I.B.V.S. No. 2793 26. This paper. The (O-C) residuals in the Table I were computed with the following light elements given in the Second Supplement to the Third Edition of GCVS (1974): Hel MinI JD = 2440438.383 + 1d.146082.E. (1) The (O-C) values are plotted in Figure 1. [FIGURE 1] Figure 1. O-C diagram of AT Pegasi. The plusses, circles, dots and triangles denote visual, photographic, photoelectric primary and photoelectric secondary minima, respectively, The dashed and solid lines represent the computed parabola using the equation (2) and (3), respectively. Using all times of primary minima listed in Table I, quadratic light elements have been calculated by the weighted least squares method as follows: Hel MinI JD = 2440438*3946 + 1d.1460884.E - 1d.00*10E-9.E^2. (2) +-14 +-3 +-10 This means that the period of AT Peg is decreasing with the rate of (5.5 +- 0.5) second per century. On the other hand, using only all of the photoelectric times of minima, following quadratic light elements have been obtained with the same method: Hel MinI JD = 2440438.3898 + 1d.1460938.E - 1d.92*10E-9.E^2. (3) +-14 +-10 +-21 The decrease in the period is (10.6 +- 1.1) second per century for this set. The light elements given by the equation (3) are in good agreement with the photoelectric times of minima but, the photographic and visual observations show large deviations. However, the light elements given by the equation (2) seem to fit all observations. At present, it is difficult to decide which elements are appropriate because of the lower accuracy of the photographic and visual observations. For this reason, the photoelectric minimum times are needed. The light and colour curves of AT Peg are shown in Figure 2 where the phases have been calculated with the following linear light elements: Hel MinI JD = 2445615.2541 + 1d.1460766.E. (4) +-4 +-3 These light elements have been obtained with the photoelectric primary minima between the years of 1975 and 1985 using the least squares method. The light curves of AT Peg show a deep primary and a shallow secondary minimum. Their amplitudes are about 0m.845 and 0m.785 at the primary, 0m.080 and 0m.100 at the secondary minimum in B and V light, respectively. The system is redder at the primary and slightly bluer at the secondary minimum which implies that the spectral type of the secondary is later than the primary. The photometric analysis of the light curves is in progress and will be published elsewhere. This work has been partly supported by the Research Foundation of Ege University with the project number 1985/036. [FIGURE 2] Figure 2. The light and colour curves of AT Peg. N. GUDUR, C. SEZER and O. GULMEN Ege University Observatory Campus P.K. 21 Bornova, Izmir - Turkey References: Cristaldi, S. and Walter, K.: 1963, Astron. Nachr. 287, 103. [BIBCODE 1963AN....287..103C ] Giuricin, G., Mardirossian, F. and Predolin, F.: 1981, Astron. Astrophys. Suppl. Ser. 43, 251. [BIBCODE 1981A&AS...43..251G ] Hill, G. and Barnes, J.V.: 1972, Publ. Astron. Soc. Pacific, 84, 430. [BIBCODE 1972PASP...84..430H ] Margrave, T.E.: 1981, Inform. Bull. Var. Stars, No. 1930.