COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1285

                                                  Konkoly Observatory
                                                  Budapest
                                                  1977 May 27


               ON THE PERIOD OF SW Lac

SW Lac is one of the well observed W UMa-type stars. The
average lengths of period of this star were given by Purgathofer
and Prohazka (1966):

                   Table 1

Years            Period        Change of    DeltaP
                                period
1893-1914        0.3207124d
1914-1928        0.3207151  JD 2419990      +0.23s
1928-1951        0.3207166     2425210      +0.13
1951-1960        0.3207213     2433690      +0.41
1960-1968        0.3207282     2436755      +0.60
-------------------------------------------------
1973-            0.3207144                  -1.19

   It can be seen from Table 1 and from the O-C curve (Fig. 1)
that the period was increasing in the years 1893-1968. The analysis 
of the photoelectric times of minima from the years 1969-
1976 leeds to the conclusion that the period of SW Lac decreased 
1.2 sec in the years 1969-1973.
   The possible reason of this decrease of the period may be:
1. The O-C curve has a periodical nature with a period of about
   80 years and the changes in the period of SW Lac might be due to
   the existence of a third component, as it was mentioned by Schilt
   (1923) - but there is no evidence for a third body in this system.
2. The changes of period are attributed to the instrinsic variability 
   of SW Lac, mainly to the active mass exchange processes
   in the system.
   Further photoelectric observations of this interesting system 
and determinations of times of minima would be highly 
desirable.

                       [FIGURE 1]

   Predictions of times of minima for the near future are
given by the formula:

           Min. I =JD 244 2697.404 + 0.3207144d E.


      ZOFIA FRASINSKA              J.M. KREINER

   Astronomical Observatory      Silesian University
   of the Jagiellonian           Institute of Physics
   University                    Astronomical Laboratory
   Cracow, Poland                Katowice, Poland



References:

Purgathofer, A., Prohazka, F. 1966 Mitt. Wien 13, 151
Schilt, J. 1923 B.A.N. 2, 1 [BIBCODE 1923BAN.....2....1S ]