COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1991 Konkoly Observatory Budapest 1981 July 14 HU ISSN 0374-0676 LIGHT VARIABILITY OF NOVA DELPHINI 1967 IN 1980 We observed Nova Delphini 1967 (= HR Del) during 7 nights in 1980 at the Hamburg Observatory in Bergedorf, The 1.2 m (f/13) Ritchey - Cretien telescope and a pulse counting photometer - polarimeter with two RCA C 31034 photomultipliers (Schroder, 1978) was used; we measured in an instrumental system close to the Johnson V-band (Schott 2 mm GG 495 + 1 mm VG 6 + 2 mm BG 38) and integrated for about 40 seconds. Comparison stars Nos. 5 and 6 of the list of Barnes, Evans (1970) were adopted as local standards and served also for determining the extinction. The nova has been monitored for 22.2 hours, but for the detailed analysis of the light curve we used only observations made in the best atmospheric conditions (15 hours in 5 nights - Table I), The observed brightness varied slightly (Fig. 1) and could be approximated by a one-cycle sinusoid, Six extrema could be derived from the light curve of three nights (Table I) whereas only an increase in brightness was observed on Sep. 3/4 and a brightness decrease appeared on Sep. 7/8. TABLE I. Journal of observations and extrema of the light curve. Date Period Number MIN MAX 1980 (UT) of obs. JD_hel 2444400 + Aug.14/15 21:23- 00:50 40 66.424 66.512 Sep.1/2 19:43- 00:03 50 84.441 84.349 Sep.3/4 21:25- 23:12 18 -- -- Sep.7/8 19:58- 21:09 18 -- -- Sep.15/16 19:12- 83:07 37 98.327 98.436 [FIGURE 1] Fig. 1 The light curve of N Del 1967 in three nights in 1980; v = v(Nova) - v(Comp.6). Assuming a periodic variation we looked for solutions in the range of 0.14d - 0.27d. Unfortunately, due to rather scarce observations, there exist numerous periods with a reasonable fit to our extrema. We eliminated most of the periods because they were not compatible with the estimated phase of the light curve on Sep. 3/4 and 7/8, respectively, The possible periods are as follows: 0.152706d, 0.165363d, 0.180308d, 0.198222d, 0.220089d. +- 41 +-37 +-33 +-35 +- 53 Similar to the method we had used for reducing our 1977 and 1979 data (Kohoutek, Pauls, 1980) we computed a least-squares fit of our Delta v = v(Nova) - v(Comp.6) measurements to a sinusoid. We selected only periods close to the possible periods given above and found the results summarized in Table II: t_0 is the time of the phase 0.0, A_v is the half-amplitude. It is not possible to find out the true period from our photometric observations in 1980. The reduced chi-square value Chi^2, ( 0.025 mag was adopted as an estimated error of a single Delta v measurement) excludes the period near 0.1527d and is not in favour of 0.165339d, but the difference in Chi^2. is small between the remaining three periods given in Table II. In Fig. 2 we present the mean Delta v light curve of N Del 1967 corresponding to the period 0.180289d. TABLE II Mean light curve of N Del 1967 in August - September 1980. Sinusoidal fit using 163 Delta v measurements. Period (d) 0.165339 0.180289 0.198207 0.220071 m.e. +- 34 38 44 54 Chi^2 1.216 1.097 1.030 1.016 t_0 (d) 66.3713 66.3619 66.3514 66.3402 ( JD_Hel- 2444400) +- 44 44 47 53 A_v (mag) 0.037 0.039 0.039 0.039 m.e. +- 3 3 3 3 [FIGURE 2] Fig.2 Mean light curve of N Del 1967 in 1980; p=0.180289d, n=163. The best periods resulting from the 1979 observations were 0.1776d and 0.2167d (slightly poorer fit). In 1980 we obtain periods somewhat larger, 0.18029d and 0.22007d (slightly better fit and an additional period 0.19821d, The period might increase by about 0.003d within a year, but this vague proposition has to be proved. It should be added, that there also exists a solution P = 0.17834d (Chi^2 = 1.142) much closer to one of the periods found in 1979, but the predicted phase of the photometric light curve for Sep. 7/8 is inconsistent with the observed brightness decrease of the nova. The decrease of the semi-amplitude of the light curve from 0.084 mag (1977) and 0.051 mag (1979) to 0.039 mag (1980) seems to be guaranteed. Further photometry of this nova is planned, We wish to thank Dipl. Phys. Th. Kleine for providing us with some computer programmes. L. KOHOUTEK R. PAULS H.-M. STEINBACH Hamburger Sternwarte D-2050 Hamburg 80 Federal Republic of Germany References: Barnes, T.G., Evans, N.R., 1970, Publ.Astron.Soc.Pacific 82, 889 [BIBCODE 1970PASP...82..889B ] Kohoutek, L., Pauls, R., 1980, Astron.Astrophys, 92, 200 [BIBCODE 1980A&A....92..200K ] Schroder, R., 1978, Mitt.Astron.Ges. 43, 295 [BIBCODE 1978MitAG..43..295S ]