COMMISSIONS 27 AND 42 OF THE IAU
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 3853

                                             Konkoly Observatory
                                             Budapest
                                             2 March 1993

                                             HU ISSN 0374-0676


            TIME OF LIGHT MINIMUM OF BW VULPECULAE^1


BW Vul (HD 199140 = HR 8007, B2III, V = 6.55) has the largest known amplitude of
light variation and radial velocity variation among the beta Cephei stars.  The
period of the variation is approximately 5h, and is now increasing at a rate of
about 2 seconds/century.

We report photometric observations obtained on September 26, 1992 at
Jungfraujoch Observatory with a 76 cm reflector telescope and the Geneva P1
photoelectric photometer.  A Lallemand S-11 photomultiplier, refrigerated at
about -23degC, was used as detector.  The system was equipped with a DC
amplifier and a strip-chart recorder.  All measurements were taken through the
V, filter of the Geneva system.

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).

The observations were carried out recording to the scheme C_1, BW Vul, C_1, BW
Vul, C_1, BW Vul ...  and, in addition, a measurement of C_2 was taken at the
beginning and at the end of the complete observing sequence.  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 kv_1 = 0.569 was derived by
application of the classical Bouguer method on the measurements of C_1.  The
mean magnitude difference between C_1 and C_2 (in the sense C_1 minus C_2) was
-0.754 +- 0.003m.  Considering the rather large value for the extinction
coefficient, and also the fact that the measurements had to be stopped for
cirrus clouds, we assess the quality of our data as of weight 2-3 on the scale
given by Sterken et al.  (1993).  Table 1 gives the differential V1 magnitudes
BW Vul minus C_1

The time of minimum light Tmin = HJD2448892.3949 was derived using the method
outlined by Sterken et al.  (1987).  The residual to the linear ephemeris given
by Sterken et al.  (1993) equals -0.0038d.  Figure 1 shows all Tmin values
obtained since June 15, 1988 (that is, our new Tmin and also those taken from
Table 1 of Sterken 1993).

Our result indicates that probably no abrupt change in the period of BW Vul has
occurred in the last year.  However, more photometric data are needed to
elucidate how much of the forthcoming changes of the period of BW Vul can be
ascribed to the light-time effect in a binary system.  For a detailed
discussion on the interpretation of the period changes in BW Vul, we refer to
Sterken (1993).


                            [FIGURE 1]

 Figure 1: O-C diagram for all available times of minimum since June 15, 
 1988 (the solid circle represents the new time of minimum reported in 
 this paper) according to the cycle-count scheme given by Sterken (1993) 
 and with P=0.2010443d and T0=2447328.4751d.


                    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_2
     8892.3482  0.130   8892.3602   0.169
     8892.3644  0.201   8892.3686   0.209
     8892.3779  0.228   8892.3818   0.221
     8892.3866  0.233   8892.3906   0.229
     8892.3946  0.234   8892.3984   0.232
     8892.4028  0.225   8892.4087   0.222
     8892.4130  0.218   8892.4182   0.216
     8892.4224  0.198   8892.4262   0.192
     8892.4302  0.189   8892.4339   0.160
     8892.4375  0.153   8892.4412   0.133

This work was supported by a research grant from the Belgian Fund for
Scientific Research (NFWO).


                          C. STERKEN
                          (National Fund for Scientific Research)
                          University of Brussels
                          Pleinlaan 2, B-1050 Brussel
                          Belgium

References:

Sterken C., Snowden M., et al., 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., Pigulski, A., and Liu Zongli; 1993; Astron. Astrophys. Suppl. Ser.,
   in press [BIBCODE 1993A&AS...98..383S ]
Sterken C., 1993, Astron. Astrophys., in press [BIBCODE 1993A&A...270..259S ]

^1 BASED ON OBSERVATIONS COLLECTED AT THE HOCHALPINE FORSCHUNGSSTATION
JUNGFRAUJOCH   (SWITZERLAND)