COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1348 Konkoly Observatory Budapest 1977 October 11 A VARIABLE LIGHT-CURVE IN AU MONOCEROTIS New photoelectric observations in V light of the eclipsing binary AU Monocerotis were carried out during the period September 1976 - April 1977 at the Observatory of Torino with a 45 cm reflector by means of a photoelectric photometer equipped with a photomultiplier 6256 S EMI 1). These observations, including three minima, show that the cyclic variation of the period suggested in a previous paper (Lorenzi, 1977) and remarked by Todoran (cf. IBVS No. 1217), is not real, but rather the light-curve of this binary is shifted up and down, probably because of long period variations of luminosity of the system as a whole. In order to eliminate any possible fluctuation due to the comparison star, three comparison stars (see Table 1) were used and checked with respect to each other (Fig. 1). It turned out that the star Table 1 No. mV (SAO) SP 1 BD -1d1424 9.0 B8 2 BD -1d1447 8.6 K0 3 BD -1d1413 9.0 B9 No. 1, already utilized in the previous work (1977), has shown some instability. Though this instability is very small (see Fig 1a, 1b), star No. 3 was preferred as comparison. A mean value of the extinction coefficient K_V was determined each night and the corresponding corrections applied to all observed points; these have been reduced to the international UBV system with the determination of the instrumental scale factors (Hardie, 1962). 1) The tables of the individual photoelectric observations are available at the Observatory of Torino (Italy). [FIGURE 1] Figure 1: Mean values of the differences in V magnitude, corrected for differential extinction, for each pair of comparison stars obtained during the single nights of observation. [FIGURE 2] Figure 2: Three observed minima (triangle) refer to minimum No. 1, (dot) to minimum No. 2, (square) to minimum No. 3. The results obtained till now suggest that the light-curve might be subject to long period variations. For three minima (Fig. 2) the times of minimum light and their mean error are computed by the KW method (Kwee and van Woerden, 1956). The results are listed in Table 2. These epochs are in quite good agreement with the linear ephemeris (Lorenzi, 1977): M (E) = 2442801.3602 + 11.11306d E. (1) Table 2 J.D. Hel sigma_KW 1 2443123.6530 +-0.0006d 2 3190.3323 .0005 3 3201.4495 .0011 Observations carried out in other phases apparently confirm the variability of the light-curve. Consequently, in order to ascertain the reality of this phenomenon, observations in the same ranges of phase were repeated. The results of this research are collected in Table 3. Table 3 N Phase DeltaV s_DeltaV 19/9/1976 46 0.5303 -0.715 +0.001 25/11/1976 21 .5390 - .799 .003 5/12/1976 34 .5315 - .770 .004 4/ 3/1977 21 .5307 - .650 .003 15/ 3/1977 42 .5232 - .627 .003 2/ 9/1976 14 .0019 + .048 .002 19/11/1976 18 .0019 - .037 .002 11/12/1976 15 .0019 - .046 .004 15/ 2/1977 18 .0019 + .029 .005 26/ 2/1977 16 .0019 + .022 .003 11/12/1976 18 .9958 - .079 .006 15/ 2/1977 13 .9958 - .013 .007 26/ 2/1977 16 .9958 + .004 .002 17/10/1976 26 .0400 - .657 .005 12/ 4/1977 33 .0400 - .508 .005 28/11/1976 26 .9023 - .815 .002 8/ 3/1977 14 .8942 - .643 .012 All data refer to normal points. The columns contain respectively: date, number of single observations used to form a normal point, phase according to ephemeris (1), DeltaV according to comparison star No. 3, mean error s_DeltaV. Their analysis suggests a longterm displacement of the whole light-curve. A series of observations carried out at maximum during consecutive nights, compared with those of December 1976, seem to confirm the slow variation of the light-curve. Table 4 summarizes these results, following the same scheme as Table 3. Table 4 N Phase DeltaV s_DeltaV 13/12/1976 14 0.2462 -0.873 +-0.004 13/ 4/1977 17 .1312 - .615 .004 75/ 4/1977 15 .3107 - .665 .403 16/ 4/1977 15 .4006 - .651 .005 Now we should like to recall that the conclusions drawn in a previous paper were based on the assumed hypothesis (Lorenzi, 1977; see footnote 2) that the light-curve would be constant with time. This assumption was justified by the sole existing photographic light-curve (Wachmann, 1954); as a matter of fact, it was built up collecting observations spread through a period of about fifteen years and the probable fluctuations were smoothed by plotting the mean values. This possible shift of the whole light-curve alongside the vertical axis, due to a long period fluctuation in the luminosity of the system, rules out the procedure of deducing the minimum epochs from parts of the ascending or descending branches through an extrapolation alongside the phase axis. Consequently, the epochs Nos. 16-78-19 reported in Table 1 of the previous paper (Lorenzi, 1977) are not real, since they were deduced from the remarkable shifting of the ascending branches with respect to the minimum No. 17. Possibly, owing to the great difficulties in the observation of this system, some previous epochs of minimum have also been deduced in the same way or from normal minima, originating the scattering of the (O-C)'s. M. CERRUTI-SOLA L. LORENZI Osservatorio Astronomico di Torino 10025 Pino Torinese, Italia References: Hardie, R.H.: 1962, Stars and Stellar Systems, Chicago Press, Vol. II, 178 Kwee, K.K., Woerden, H. van: 1956, Bull.Astron.Inst.Neth. 12, 327 [BIBCODE 1956BAN....12..327K ] Lorenzi, L.: 1977, Astron.Astrophys. 55, 195 [BIBCODE 1977A&A....55..295L ] Todoran, I.: 1976, I.B.V.S. No. 1217 Wachmann, A.A.: 1954, Astron.Abh.d.Hamb.Sternwarte 7, 407 [From IBVS 1363] ERRATUM In I.B.V.S. No. 1348 one of the references was incorrectly printed. It should read : Lorenzi, L.: 1977, Astron Astrophys. 55, 295