COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2609 Konkoly Observatory Budapest 19 October 1984 HU ISSN 0374 - 0676 A NEW VARIABLE STAR (No. 218) NEAR THE GLOBULAR CLUSTER NGC 6401 * Recent collaboration with W. and A. Wehlau (University of Western Ontario), to determine the periods of some of the 217 variable stars previously identified in and around the globular cluster NGC 6401 (Terzan and Rutily, 1971, 1973) led me to make further photometric measurements of the 52 R plates (Eastman Kodak 103 aE + Ilford filter 204, lambda_eff ~= 6500 A) which I had obtained from 1968 to 1977 at the Newtonian foci of the 80 and 193 cm telescopes of the Observatoire de Haute-Provence. Results on the periods found will be published in the near future. However, in the course of those photometric measurements (using the iris-diaphragm photometer Sartorius), I found a new variable, the 218th, which was previously unknown (Kholopov, 1983). Its equatorial and galactic coordinates for the equinox 1950.0 are : alpha = 17h36.89m l = 3.49d delta = -24d01.84' b = 3.64d This star is located about 20' southeast of the cluster NGC 6401 (Figure 1), which places it outside the field of the plates taken with 193 cm telescope. It is, however, clearly visible on Palomar Observatory Sky Survey charts, O-172 and E-172. Since the plates for these charts were taken on the same night, they have been used to estimate the color index, CI_B-R >= 3 mag. * based on observations collected at the Observatoire de Haute- Provence and Observatoire de Calern [FIGURE 1] Figure 1. Finding chart (in R) for the variable star No. 218 [FIGURE 2] Figure 2. Light curve for the variable star No. 218 Table I -------------------------------------------------- Observations Plate J.D. m_R Number No (+2439500.00) -------------------------------------------------- 1 1320 471.55 16.8 2 1329 472.58 16.8 3 1341 476.53 16.8 4 1345 477.52 16.7 5 1346 478.55 16.7 6 1347 478.58 16.7 7 1350 479.59 16.7 8 1572 1205.57 16.8 9 1582 1206.57 16.8 10 1592 1208.58 16.9 11 1740 1590.55 15.9 12 1744 1594.61 16.1 13 1751 1596.62 16.2 14 1754 1597.50 16.3 15 1756 1599.55 16.2 16 1773 1601.57 16.4 17 1875 1944.61 14.0 18 1883 1945.64 13.9 19 1891 1946.60 14.0 20 1919 1951.58 14.0 21 1978 2329.48 14.9 22 1981 2329.57 14.9 23 1987 2332.49 14.7 24 1990 2332.58 14.7 25 2002 2336.49 14.4 26 2290 2720.50 16.6 27 2573 3745.63 15.9 28 2588 3747.62 15.7 29 2596 3748.62 15.6 30 2605 3749.61 15.5 31 2644 3759.62 14.5 32 CE 1072 6379.49 16.5 33 CE 1077 6380.51 16.4 34 CE 1085 6381.48 16.3 35 CE 1088 6384.45 16.2 36 CE 1092 6385.38 16.2 37 CE 1096 6385.51 16.2 ---------------------------------------------- The light curve shown in FIGURE 2 was derived from measurements of 31 good plates taken with the 80 cm telescope and from 6 plates obtained this summer with the CALERN/Schmidt telescope (INAG-CERGA). The values of mag_R with the corresponding Julian dates are given in Table I. The prefix CE (CERGA) in font of the numbers of the last six plates indicates their origin. The amplitude, A >=2.9 mag_R, suggests that this star is a long period variable with epochs of maximum given by: T_max = 2,439,618+203.2 E +- 0.1 While the period is well determined, it is difficult to estimate the accuracy of the epoch of maximum light because of the lack of points on the light curve between 0.6 and 0.8. The interstellar absorption in V and B to the cluster NGC 6401 may be estimated using A_V = 3E_B-V and A_B = 4E_B-V with the following values : Woltjer, 1975 E_B-V = 0.5 a relatively low value. Bernard, 1976 E_B-V = 0.85 A_V = 2.55 mag A_B = 3.4 mag Harris, 1976 E_B-V = 0.79 A_V = 2.47 mag A_B = 3.16 mag So, adopting A_B = 3.25 mag ( a value comparable to A_pg 5.5 mag, Terzan, 1965, in front of the bright cloud B of Sagittarius, in the direction of the star 45 Oph, which is one of the most observed zones in B) one could derive the spectral type of the star and use it as an argument to test our hypothesis that star No. 218 is a long period variable if the colour index (B-V) were available. Consequently, in the near future, I plan to establish B and V magnitude sequences (mag_B = 20,mag_V ~= 19) in the field of the globular cluster NGC 6401 and to determine the B - V color of this variable, taking into account the interstellar absorption in this direction towards the center of our galaxy. I express my thanks to Mr. and Mrs. A. Wehlau for discussion of this work. A. TERZAN Observatoire de Lyon 69230 St-Genis-Laval (France) References: Bernard, A., 1976: Astron. Astrophys. Suppl. Ser., 25, 281. [BIBCODE 1976A&AS...25..281B ] Harris, W.E., 1976: Astron. J., 81, 1095. [BIBCODE 1976AJ.....81.1095H ] Kholopov, P.N., 1983: private communication Terzan, A., 1965: Ann. Astrophys., 28, 935. [BIBCODE 1965AnAp...28..935T ] Terzan, A., and Rutily, B., 1971: Astron. Astrophys., 16, 408. [BIBCODE 1972A&A....16..408T ] Terzan, A., and Rutily, B., 1973: Variable Stars in Globular Clusters and in Related Systems, p. 68, ed. J.D. Fernie. [BIBCODE 1973ASSL...36...68T ] Woltjer, L., 1975: Astron. Astrophys., 42, 109. [BIBCODE 1975A&A....42..109W ]