COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1386

                                    Konkoly Observatory
                                    Budapest
                                    1978 February 10




     ON THE PERIOD CHANGE OF THE BETA CMa VARIABLE BW Vul




   Beta CMa variable BW Vul was observed photoelectrically in
uvby system at EGE University Observatory during the summer season 
of 1977. HD 198820 was used as the comparison star. The 
observations were carried out with the 48 cm Cassegrain telescope
equipped with an unrefrigerated 1P21 photomultiplier.

     Valtier (1976) has given the period of this star as:
         P = 0.2010298 + 5.902 x 10^10 t
and the ephemeris as:
      Max = 2427999.9574 + 0.2010298 E + 5.932x10^-11 E^2.
 This corresponds to rate of increase of about 1.86 s/cent. A
year ago, Cherewick (1975) had also found the ephemeris as:
     Max = 2428802.670 + 0.201032 E + 6.04x10^-11 E^2.
They admitted that the (O-C) residuals exhibit a parabolic curve
using a period P_0.
     The new maximum times together with those given by Valtier
(1976) were used in computations.

   Using the period P_0=0.20103 and the epoch T_0=2434247.8210,
the (O-C)_I residuals have been plotted against time. At first, we assumed
that a parabolic curve can be fitted to the (O-C)_I residuals and
therefore computed the period and ephemeris as:

            P = .20103278 + 8.129x10^-10 t
     Max = 2434247.8214 + 0.20103278 E + 8.1714x10^-11 E2.
The rate of increase is about 2.56 s/cent.
   However, when we reexamined the (O-C)_I variation, we found
that it would be necessary to compute a new period and ephemeris for
the reason that the (O-C)_I residuals did not in fact show a 
parabolic form, but a broken line (see Fig.1). The first segment of
the broken line lies between JD 2433750 and 2441400 approximately, 
while the second part from JD 2441400 to the present

                      [FIGURE 1]

   Therefore we assumed two linear variations for (O-C)_I
values and computed the following results:   
For the first segment (from JD 2433750 to JD 2441400 approx.)
     Max = 2434247.8215+ .201035135 E            
     Correlation coefficient r= .9981
For the second segment (from JD 2441400 to the present time)
     Max = 2441537.7724+ .20104071E
     Correlation coefficient r= .9938
     We can assume that the period was constant up to the years
1970-71, but afterwards showed a sudden increase and now stays
stable.



                           ZEYNEL TUNCA
                      EGE University Observatory
                            Izmir,Turkey




References:
Cherewick, T.P., Young, A. 1975, Publ.Astron.Soc.Pacific 87, 311 [BIBCODE 1975PASP...87..311C ]
Valtier, J.C. 1976, A.Ap. 51, 465  [BIBCODE 1976A&A....51..465V ]