COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3901 Konkoly Observatory Budapest 28 June 1993 HU ISSN 0374 - 0676 TIME OF LIGHT MAXIMUM OF BW VULPECULAE^1 We report photometric observations of the beta Cephei star BW Vul (HD 199140 = HR8007, B2III, V = 6.55) obtained on June 17 and 18, 1993 at Jungfraujoch Observatory. The measurements were obtained in the V_1 filter band of the Geneva photometric system. The comparison stars were the same as C_1 and C_2 used by Sterken et al. (1986), viz. HD 198820 = HR 7996 (B3III, V = 6.44) and HD 198527 = SAO 089185 (B9.5V, V = 7.0). On June 17, the observations were carried out according to the scheme C_1, BW Vul, C_2,C_1, BW Val, C_2, C_1 BW Vul, . . . and on June 18 the scheme C_1, BW Vul, C_1 , BW Vul, C_1, BW Vul, C_1,. . . was adopted, with additional measurements of C_2 during the first and last hours of the night. Each datapoint consisted of a measurement of about 1 minute duration. Sky background was measured about once every two cycles. The data were corrected for sky background contribution, and for the effect of atmospheric extinction; the extinction coefficient k_(v_1), (0.256 and 0.183, respectively on June 17 and 18) was derived by application of the classical Bouguer method on the measurements of C_1 and C_2. The mean magnitude difference between C_1 and C_2 (in the sense C1 minus C_2) was -0.754m +- 0.003m for the first night, and -0.761m f 0.001m for the second. We assess the quality of our data as of weight 4 (on the scale given by Sterken et al. 1993) for June 17 and of weight 5 for June 18. Table 1 gives the differential V_1 magnitudes BW Vul minus C_1. Sterken et al. (1987) argued that there are two reasons for using time of minimum instead of time of maximum for determining the variation of the pulsation period of BW Vul. First, due to the occurrence of the stillstand phenomenon (until about 30 minutes before the time of maximum light), the time base of data suitable for determining the time of maximum is more than two times smaller than it is at minimum light. Second, the disturbance of the stillstand on the shape of the light curve progressively increases towards infrared wavelengths (see Fig. 3 of Sterken et al. 1987), a circumstance that may seriously affect the result when data obtained with different filter systems are combined. However, due to the coincidence of the phase of minimum light and the end of astronomical twilight, and due to the very short duration of the night at this time of the year, no minimum of the light curve could be observed, but we did cover one phase of maximum on June 18, which yielded a time of maximum light T_max = HJD2449157.4855. The linear ephemeris given by Sterken et al. (1993) is not directly applicable, so we calculated the O - C values on the basis of the period P = 0.2010443d derived by these authors, and on the times of maximum given in Table 6 of their paper. That O - C 1 BASED ON OBSERVATIONS COLLECTED AT THE HOCHALPINE FORSCHUNGSSTATION JUNGFRAUJOCH (SWITZERLAND) diagram (To = 2428801.9530d +- 0.0007d) is given in Fig. 1, the position corresponding to our new time of maximum is indicated by a solid circle. [FIGURE 1] Figure 1: O-C diagram for all available times of maximum since June 15, 1988 (the solid circle represents the new time of Maximum reported in this paper) according to the cycle-count scheme given by Sterken (1993) and with P=0.2010443d. Table 1.Differential V_1 magnitudes of BW Vul minus HD 198820. HJD is heliocentric julian date minus 2,440,000. HJD Delta V_1 HJD Delta V_1 HJD Delta V_1 HJD Delta V_1 9156.4806 0.017 9157.3936 0.224 9157.4655 0.075 9157.5124 0.106 9156.4901 0.047 9157.3992 0.212 9157.4707 0.056 9157.5155 0.112 9156.4965 0.072 9157.4047 0.191 9157.4745 0.046 9157.5239 0.141 9156.5028 0.089 9157.4102 0.186 9157.4778 0.040 9157.5324 0.163 9156.5083 0.114 9157.4158 0.166 9157.4808 0.028 9157.5370 0.175 9156.5140 0.123 9157.4216 0.145 9157.4845 0.024 9157.5427 0.194 9156.5393 0.190 9157.4272 0.125 9157.4874 0.027 9157.5472 0.210 9156.5455 0.208 9157.4322 0.112 9157.4906 0.029 9157.5507 0.219 9156.5511 0.217 9157.4377 0.101 9157.4940 0.045 9157.5571 0.234 9156.5583 0.236 9157.4445 0.091 9157.4976 0.050 9157.5612 0.241 9156.5636 0.240 9157.4511 0.093 9157.5003 0.064 9157.5652 0.239 9156.5686 0.235 9157.4545 0.086 9157.5033 0.079 9156.5768 0.236 9157.4586 0.087 9157.5063 0.082 9137.3861 0.235 9157.4619 0.085 9157.5097 0.098 The "annual" mean O - C values for the times of maximum yield an estimated mean error on one T_max of 0.0027d, whereas the estimated mean error on one T_min, amounts to about 0.0017d. Our result indicates that no strong change in the period of BW Vul has occurred since September 1992. Evidently, more photometric data are needed to monitor the forthcoming changes of the period of BW Vul. For a detailed discussion on the interpretation of the period changes in BW Vul, we refer to Sterken (1993). This work was supported by a research grant from the Belgian Fund for Scientific Research (NFWO). The authors are indebted to Observatoire de Geneve for the permission to use the Geneva photometer. C. STERKEN^1 K. VYVERMAN Astronomy Group University of Brussels Pleinlaan 2, 1050 Brussels, Belgium H.W.W. SPOON SRON, Utrecht laboratory Sorbonnelaan 2, NL-3584 CA Utrecht The Netherlands References: Sterken C., Snowden M., Africano J., Antonelli P., Catalano F.A., Chahbenderian M., Chavarria C., Crinklaw G., Cohen H.L., Costa V., de Lara E., Delgado A.J., Ducatel D., Fried R., Fu H.-H., Garrido R., Gilles K., Gonzalez S., Goodrich B., Haag C., Hensberge H., Jung J.H., Lee S.-W., Le Contel J.-M., Manfroid J., Margrave T., Naftilan S., Peniche R., Pena J.H., Ratajczyk S., Rolland A., Sandmann W., Sareyan J.-P., Szuskiewicz E., Tunca Z., Valtier J.-C., and vander Linden D., 1986, Astron. Astrophys. Suppl. Ser., 66, 11 [BIBCODE 1986A&AS...66...11S ] Sterken C., Young A., and Furenlid I.,1987, Astron. Astrophys., 177, 150 [BIBCODE 1987A&A...177..150S ] Sterken C., 1993, Astron. Astrophys., 270, 259 [BIBCODE 1993A&A...270..259S ] Sterken, C., Pigulski, A., and Liu Zongli, 1993, Astron. Astrophys. Suppl. Ser., 98, 383 [BIBCODE 1993A&AS...98..383S ] 1 National Fund for Scientific Research