COMMISSION 27 OF THE I.A.U.
                 INFORMATION BULLETIN ON VARIABLE STARS
                              Number 2964

                                                         Konkoly Observatory
                                                         Budapest
                                                         5 December 1986

                                                         HU ISSN 0374 - 0676


                 A NEW PERIOD CHANGE OF SW LACERTAE

SW Lac (BD+37d4717) is a well-known W UMa-type binary with occasional
period changes (see e.g. Mikolajewska and Mikolajewski, 1981).
   UBV photometry of SW Lac was carried out during ten nights between 29
June and 14 July, 1986. The observations were made with the 74-inch reflector
at the Kottamia Observatory, Egypt. Two primary and three secondary minima
were observed during that time. Table I contains the times of minima, the
number of cycles and the O-C values as calculated from the ephemeris given
by Bookmyer (1965):

         H.J.D.=2437572.57231+0d.32072811*E


               Table I

    H.J.D.        E          O-C

2446613.5361    28189.5    -0d2013
    614.4973    28192.5    -0.2023
    619.4704    28208      -0.2004
    620.4321    28211      -0.2009
    624.4404    28222.5    -0.2017

   The Figure shows the O-C diagram of SW Lac covering 25 years (from E=0).
The points in the Figure were taken from Lafta and Grainger (1985; and 
references therein), Niarchos (1985), Evren et al. (1985), and Table I. The 
points represent averages when there have been more than one O-C values 
during 100 cycles.
   Lafta and Grainger (1985) used all the then available photoelectric
minima in constructing their diagram, and they used Bookmyer's ephemeris.
In order to make the comparison of the two O-C diagrams easier, the same
ephemeris was used in this paper, too.
   The right hand side of the Figure shows that SW has had another period
change since 1985 (E ~27100). The exact time of the change is uncertain
because of the large gap between the O-C values of Evren et al. (1985) and
those in the present paper. A new period can, however, be determined from
the Figure using the last two sets of data: P = 0d.320706586

                            [FIGURE 1]

   Frasinska and Kreiner (1977) listed the periods of SW Lac since 1893.
Table II shows an updated version of their Table I.


                           Table II

  years         Period      DeltaP                 Ref.

1893-1914      0d3207124                 Frasinska and Kreiner (1977)
1914-1928      0.3207151    +0s23                  - " -
1928-1951      0.3207166    +0.13                  - " -
1951-1960      0.3207213    +0.41                  - " -
1960-1968      0.3207282    +0.60                  - " -
1973-1977      0.3207144    -1.19                  - " -
1977-1979      0.3207229    +0.73           Evren et al.(1985)
1980-1985      0.3207198    -0.24                  - " -
               0.3207204                      Niarchos (1985)
1986-          0.3207066    -1.1               present paper

Frasinska and Kreiner (1977) mentioned two possible causes for this period
change: (i) the presence of a third body in the system, (ii) intrinsic 
variations due to mass exchange processes in the system. As there is little 
evidence for periodicity in the period change, the second explanation 
seems more probable.


                  M.A. SOLIMAN   M.A. HAMDY   H.A. MAHDY
                         Helwan Observatory
                               Egypt



References:

Bookmyer, B.B., 1965: Astron. J. 70, 415 [BIBCODE 1965AJ.....70..415B ]
Evren, S., Akan, M.C., and Ibanoglu,C., 1985: IBVS No. 2781
Frasinska, Z. and Kreiner, J.M., 1977: IBVS No. 1285
Mikolajewska, J. and Mikolajewski, M., 1981: IBVS No. 1953
Niarchos, P.G., 1985: IBVS No. 2820