COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1879

                                   Konkoly Observatory
                                   Budapest
                                   1980 December 5

                                   HU ISSN 0374-0676



  ON THE SUSPECTED ULTRASHORT PERIOD W UMa TYPE BINARIES
               BG CrA AND AB Tel *



Investigations of W UMa stars with periods shorter than 0.25d
are particularly interesting with respect to the stability
limit of such objects. CC Comae (P=0.22d) is so far the shortest
period object photoelectrically confirmed. There are, however,
still some objects listed in the fifth edition of the "Finding
List" and the "GCVS" with shorter periods. In 1980 some southern
short period W UMa stars have been observed photoelectrically
with the ESO 1m telescope and a two channel UBV photometer.
Inferior weather conditions affected the measurements reported
here and prevented a completion of the light curves. However,
the measurements are sufficient for a number of important conclusions. 
BG CrA has been observed on JD 2444440 (July 20).The
observed run of 0.25d shows only one minimum. Together with the
discovery observations by van Gent (1932), a period of approximately 
twice the value previously determined for this star has
been assumed as the best fit. The B and V light curves, shown in
Figures 1 and 2, obey the ephemeris

            2444440.5125 + 0.4446d.E.

The assignment of phase 0.50 to the observed minimum is essentially 
arbitrary. Only a part of the light curve irregularities
can be related to atmospheric influences. The general nature of
the light curve resembles a complicated semidetached binary or
magnetic active short period RS CVn stars. So BG CrA is possibly
not a W UMa system.

* Based on observations made at the European Southern Observatory,
 La Silla, Chile.


                       [FIGURE 1]

            Figure 1: B measurements of BG CrA

                       [FIGURE 2]

            Figure 2: V measurements of BG CrA



AB Tel has been discovered by Shapley et al. (1939). According
to them, its period is 0.17d, and they classified it as eclipsing 
binary of W UMa type. The object has been observed on
JD 2444435, 2444440 and 2444441 with the same instrument as
BG CrA. The measurements are shown in Figures 3 and 4.

                       [FIGURE 3]

            Figure 3: B measurements of AB Tel

                       [FIGURE 4]

            Figure 4: V measurements of AB Tel



From the minima times
           JD 2444435.5645
           JD 2444441.7580
which refer to the same phase, a period of 0.32597d has been determined. 
It is not known, however,if this minimum is a primary
or a secondary one. So again a phase assignment of 0.50 to it
is arbitrary. Obviously this object is indeed a genuine W UMa
system. The minimum shows a slightly declining profile. There
are indications that it represents an annular eclipse. From the
large amplitude it is concluded that the mass ratio of the components 
is at least 0.5.
My thanks go to the ESO staff members who assisted me in the
observations.





                          M. HOFFMANN
                      Observatorium Hoher List
                      5568 Daun / Eifel, BRD




References:
van Gent, H., 1932, Bull.Astr.Inst. Netherlands 6, 163 [BIBCODE 1932BAN.....6..163V ]
Shapley, H., Boyce, E.H., and Boyd, C.D., 1939, Harvard Ann.
          90, No. 9 [BIBCODE 1940AnHar..90..239S ]